There is no rule which relates the size of a creature with its DEX score.
On the other hand, the DM decides when the condition are appropriate for a creature trying to hide. The rat can try to hide almost everywhere due to its size, whereas a Giant Elk has a difficult time finding a suitable cover.
Also, if you watch the walk pattern of an elk and a rat, you will notice something. Rats have claws on their feet which produce a distinctive sound and a walking pattern which does not really try to hide it. Elk, on the other hand, while hoofed (very solid foot), are usually walking on soft ground and placing their hooves as gently as possible to create as little sound as possible (and attract wolves and such). An elk can sneak up behind you as you sit by the fire. A rat, on the other hand, is much more likely to be heard as it closes in. Not that many elk sneak up on people, but...
One of the most terrifying experiences is when you realize you can't hear a moose. When you are in the woods and suddenly realize a moose is probably only 50 feet from because you somehow didn't hear a 7 foot tall 1,000 lbs animal walking by you.
That said, I would agree with filcat. Give the rat Adv because it's going to be easy to hide when you're tiny... that basically giving it a +5 bonus.
I've mostly thought about this conundrum as a Druid... because scouting as a house spider is pretty easy.
As a GM it makes me almost want to turn the Druid into a "Honey I Shrunk the Kids" mini-adventure... but since everything has 1 HP it doesn't work.
There is no rule which relates the size of a creature with its DEX score.
On the other hand, the DM decides when the condition are appropriate for a creature trying to hide. The rat can try to hide almost everywhere due to its size, whereas a Giant Elk has a difficult time finding a suitable cover.
Also, if you watch the walk pattern of an elk and a rat, you will notice something. Rats have claws on their feet which produce a distinctive sound and a walking pattern which does not really try to hide it. Elk, on the other hand, while hoofed (very solid foot), are usually walking on soft ground and placing their hooves as gently as possible to create as little sound as possible (and attract wolves and such). An elk can sneak up behind you as you sit by the fire. A rat, on the other hand, is much more likely to be heard as it closes in. Not that many elk sneak up on people, but...
To back up JamesGreyWolf,
One of the most terrifying experiences is when you realize you can't hear a moose. When you are in the woods and suddenly realize a moose is probably only 50 feet from because you somehow didn't hear a 7 foot tall 1,000 lbs animal walking by you.
That said, I would agree with filcat. Give the rat Adv because it's going to be easy to hide when you're tiny... that basically giving it a +5 bonus.
I've mostly thought about this conundrum as a Druid... because scouting as a house spider is pretty easy.
As a GM it makes me almost want to turn the Druid into a "Honey I Shrunk the Kids" mini-adventure... but since everything has 1 HP it doesn't work.
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