I was looking at the opening cinematic for Baldur's Gate 3, and did some quick math. Based on the heights of people we see, the Nautiloid is several hundred feet long, the monsters are red dragons. If we assume they're not supposed to be more than adult red dragons, they're Huge (15' space) in 5e. That's bigger than dragons have been since 2nd edition D&D (in 3e they maxed out at at C, but only at great wyrm).

I don't know that I need dragons to be quite that big, and creatures do extend a bit beyond their space on the map (a human is commonly more than 5' tall..) but even if we assume that tip to tail length is twice space, that's only 30' long (and maybe 5' at the shoulder). Should Huge and Gargantuan be somewhat larger? Is there value to bringing back Colossal?

Baldur's Gate 3 is a game set in the forgotten realms, and I think they just took creative licence and tried to make everything look as cool as possible, and it works. If you have a problem with something as small as the Dragons size for a video game then i suggest you straight up just don't play it. 

I'm not having a problem with the size they have in BGIII, I'm having a problem with the size they have in D&D5e.

I feel like when you're making a monster that's, say a 25' space, it starts to strain the abstraction of monsters in 5. Considering something that size should have a reach of at least 15', just turning around to bite someone 'behind' it is equivalent to moving the head like 40 feet.

I am all for big baddies, but I'd need to homebrew the whole thing to make it satisfying. Head, claws, and tail would have different areas of reach, facing would matter, etc.

I've had a few fights with super-big things that were basically hostile terrain. Those are fun too, but unfortunately 5e doesn't handle that stuff well without some customization.

Hm. Treating giant monsters as multiple counters seems like a good idea. Imagine having Tiamat as five Large monsters (each individual head) that are required to stay within 10' of a Gargantuan monster (her body).

There's a reason why facing was removed from the game with 3rd edition.

Facing was removed for simplicity, and actually exists as a variant rule in 3.5e and an optional rule (DMG 252, in combat) in 5e. Excessive complexity is rarely a problem when dealing with a solo boss.

The big one that bugs me is the purple worm. Saw one (ok, newbie DM) whose purple wormzilla was 4x (that's 2 size categories) bigger than the MM purple worm, (player meta-whiney: even deities can't double enlarge, that violates the combined spell effects rule.) 

On top of this, he gave it a stone tunnelling speed 10x MM version, and let it make a tail and mouth attack in the same round vs the same character even although it's tail was in another county. 

Of course this was the same DM that gave slimes & oozes double the speeds of the characters.

Obviously, I'm venting over something that happened a while ago that I never felt was resolved to my satisfaction.

I think from a players standpoint, my frustration really rooted in what he called these monsters. He used names that I was familiar with so assumed I knew what I was dealing with. A lot of that frustration could have been avoided if he didn't call his homebrew versions of the monsters by the same name in the books.

But I still think a tail attack, even given a 15' melee range shouldn't be able to attack something outside of that range, even if other parts of the monster are.

The largest monster I can find in first edition is a gold dragon (54', though not specified how or if that varied with age and HD), and other large creatures were similar (purple worm 40-50', sea serpent 50'). In AD&D2 that was clarified as being size in age category 4 and a great wyrm red dragon could be more than 300' long. In D&D 3e it was again specified that length was to base of tail, and the largest dragons were C (more than 64' long), though the rules for maps were weird (a Colossal creature was 8x the length or height of a Large creature but only 3x bigger on a map).

Quote from 6thLyranGuard >>

There's a reason why facing was removed from the game with 3rd edition.

 I am in no means saying facing should be the norm. I'm not going to apply it to a goblin or to PCs. But I do think it feels appropriate for a boss monster that exceeds the size categories. As above, it's more like different monsters that are all loosely attached to each other and sharing the same pool of hp.

Solana. Seems like it would create a whole lot of extra complexity for things like flanking, movement, and area effects.

Yeah, it would, but one of the problems with solo fights is insufficient complexity.

Part of that problem is due to how many GMs run solo monsters as if they're turrets- the monster arrives, then it stands still and whomps on whatever gets in melee range with it.

Quote from 6thLyranGuard >>

Part of that problem is due to how many GMs run solo monsters as if they're turrets- the monster arrives, then it stands still and whomps on whatever gets in melee range with it.

 Avoiding surround-and-pound is a non-trivial problem for encounter design, and the options for doing all involve increasing complexity.

Making use of a behir's climbing speed, or a purple worm's burrowing ability, or simply having a goblin pay attention to the layout of the battlefield instead of just fighting in the open are still orders of magnitude easier than implementing a monster as a collection of pseudo-independent creatures.