Hello!!

I'm pretty new to DMing and had a question regarding how much a player can see during the daytime on a battlemap that takes place outside.

If they are in a field, for example, with limited obstructions like trees, how far can someone see? Also just to throw this out there, lets say the map is 400 ft by 150 ft (yes I know that is massive, its hypothetical) how much of the map should I reveal? Would you allow them to see farther if they say climbed a tree or cast fly on themselves and took off 30 ft in the air?

They can see as far as you let (want/need) them to see:)

I would not limit their vision unless there is an important gameplay reason for it. You can also always limit visibility with weather (fog, mist, precipitation…)

Realistically, they should see most of it, depending on obstacles some parts can be temporarily hidden (but I would not bother hiding stuff that is not worth revealing). And yes, based on the height of obstacles, climbing to a higher place could help them notice stuff and see previously hidden areas.

It really depends on what you want to accomplish here.

Quote from RyanPare >>

If they are in a field, for example, with limited obstructions like trees, how far can someone see?

I usually let them see the map but not all details. For example, there's a stream in the distance that I draw, but maybe thay don't see the enemies behind a tree on the other side. Perception comes into play. If the party gets closer, then passive perception, checks, or enemy actions might cause me to reveal details. There's also the case where the party might see "some enemies," but not the exact number. A rough circle might be drawn for the moment.

You might be able to see for miles, but only make out discernable details at much closer distances. Obstructions and attempts to hide will prevent all of the details of the landscape being revealed all at once. Yes, elevating one's viewing position would allow them to see further, and may provide the needed change in perspective to see new details. Slowly revealing details to the party can lead to a sense of discovery and unknown danger. 

Best answer: up to you. 

For ease of use, I will usually reveal the entire map if it is for combat. However, if creatures, unusual terrain, ruins, caves or other specific features are out of sight or too far to be seen then I won't add them to the map until they become known. In person, I'd use a non-permanent marker on some sort of surface to add these. Online, just draw them by hand or move pre-prepared elements from the GM Layer to the map layer of the VTT.

If it is for exploration and there are features they can't see at a distance then I'd describe what they see e.g. "the green canopy of the trees extends as far as you can see" or "the fields spread over rolling hills for several miles, dotted with clumps of trees, some bushes and a few rock outcroppings", then if there are features that they will find as they explore I will either describe them or put them on the map as they are discovered.

Most often though, you won't be using a detailed map for exploration unless you expect it could develop into a combat.

So, having had to adapt to online D&D due to covid I now work into my in person play sessions a VTT. 

I use https://Owlbear.rodeo or the paid and latest version https://owlbear.app which are hands down some of the most user friendly systems out there. Online everyone can access the VTT from home.

At the table Owlbear works spectacularly from a phone, but I also have a chromebook tablet that is used as a community tablet/screen for the players to visualise the battlemaps. I meanwhile have my laptop behind or as my DM screen simply because I then don't have to carry reams of paper. 

As to the rules element of this, I think the difference between player knowledge and character knowledge is important to highlight. Sometimes it's just easier to be able to have the whole of a map visible. In those cases it's essential to point out the differences between darkvision and regular vision. Darkvision allows for the characters to see as if it were bright light in dimly lit streets. In wide open fields and unlit areas however, Darkvision only allows for darkness to be seen as if dimly lit. The way I translate that to players with darkvision is to say, popping your light on in a room is the equivalent of bright light. Lighting a medium sized candle in real life is going to get you dimly lit.

In short it depends on the light available as to how far they can all see. That's not helpful I know, but is an important part of this question.

There's Visibility rules in the Dungeon's Master Guide;

Visibility Outdoors: When traveling outdoors, characters can see about 2 miles in any direction on a clear day, or until the point where trees, hills, or other obstructions block their view. Rain normally cuts maximum visibility down to 1 mile, and fog can cut it down to between 100 and 300 feet. On a clear day, the characters can see 40 miles if they are atop a mountain or a tall hill, or are otherwise able to look down on the area around them from a height.

Nociting Other Creatures: While exploring, characters might encounter other creatures. An important question in such a situation is who notices whom. Indoors, whether the sides can see one another usually depends on the configuration of rooms and passageways. Vision might also be limited by light sources. Outdoor visibility can be hampered by terrain, weather, and time of day. Creatures can be more likely to hear one another before they see anything. If neither side is being stealthy, creatures automatically notice each other once they are within sight or hearing range of one another. Otherwise, compare the Dexterity (Stealth) check results of the creatures in the group that is hiding with the passive Wisdom (Perception) scores of the other group, as explained in the Player's Handbook.

Honestly, your own real world experience will probably be more useful than anything D&D tells you. A regulation (US) football field is 300' between the goal lines and 360' between the goalposts; how much of the field do you think you can see if you enter at one end?

Note that real world terrain is rarely entirely flat and clear, and even shrub brush can reduce vision range by quite a lot.

If you’re asking for RAW, I don’t think there is anything official on how far a character can see. 
But long range on a heavy crossbow goes to 400 feet, so it seems reasonable that you can see at least that far, since you can shoot that far. And that’s not with any feats or special abilities or anything. 

It's basically up to you to allow them to see however much you want them to see. weather can be a big help to limit the group from mid maxing but when playing on owlbear rodeo I typically fog of war buildings, things they can't see around corners, etc.

A picture tells more than a thousand words. Spells and weapons have ranges. Players make choices based on those facts. Just describing the battlefield/room/dungeon is not enough. I always use two maps: one having all DM info, and one player info map.

On earth standing on completely flat ground the horizon is about 3 miles away and you can see high things much further than that. However as DM you d3cide the terrain if there is a ridge party can not see what is on the other side.

The further things are away the less detail they can see. Facial recognition starts to decline at 25ft and by 150ft is 0.  If someone is hiding in a Bush they would be harder to spot at 200ft than 10ft, how you interpret that in your game is up to you, for example you might have the hiding creature roll stealth and use that against passive perception at 60ft. Creatures with higher pp will see them from further away than that and those with lower would not see them until they are closer or not at all.

Also worth noting that climbign a tree may not always help, you'll need to decide each time. For example, in the Hobbit, when Bilbo climbs the tree, he can see much further than he could in the forest, but he can't see anything inside the forest any more, so has reduced his visibility when looking for things inside!

Than you all, appreciate the help and feedback

For combat situations I generally let them see the whole battlefield.

I use a lot of real world in some situations.  Outside I don't think your vision is restricted to so many feet.

However, creatures can be hiding behind objects like a fallen log.  They may see the log but not the creatures.

Inside combat I try and keep the areas smaller, but still don't worry if the room is lit. 

I'm relatively new also, but I've been learning a lot and one of the biggest mistakes I made early on was overreliance on a battle map.

I think it's better to adopt the mindset that the map isn't there so that you focus more on the "theater of the mind" details. It solves a lot of problems to focus on the theater of the mind and then to use the map only when combat breaks out. It's easy to forget that the map is only an approximation of what's going on.

Instead of referring to the map to determine what players see, use your imagination to think about what they should see, and then use the map to illustrate that. Just because they can see farther than your map borders, doesn't mean they see everything on the map that isn't deliberately hiding.

A player who climbs a tree or flies up in the air should be able to see farther, but it doesn't give them X-ray vision. If there was a hill or building blocking their view, sufficient altitude should allow them to see over it, but if they are in the middle of a forest, they generally will only see the tops of other trees unless a giant is making its way towards them or something.