What is the size scale and how does it work in D&D 5e? For example, how big would a huge monster be? I've been trying to scale my monsters probably, but I keep finding contradictory information and I don't own the Monster Manual. I'll try to get it when I can.
If, as an example, I have an adolescent dragon with a given wingspan, how do I know what size it should be? How much of its size is wingspan? Is the size only the height, or is it its all-around size?
There's a little table in the Monster Manual, in the first chapter that explains how stats work. It also gives the space that monsters take up on the battlefield. It's on page 6 of the book.
The spaces are as follows:
Tiny: 2.5 by 2.5ft
Small: 5 by 5ft
Medium: 5 by 5ft
Large: 10 by 10ft
Huge: 15 by 15ft
Gargantuan: 20 by 20ft or larger.
In the case of Gargantuan, the monster's description might contain more information about how big it really is. The Tarrasque for example is described as being "70 feet long and 50 feet wide" and so would probably require something like a 100 by 100ft space to fight comfortably.
This same table is also in the Player's Handbook on page 191 along with a bit of extra info on what "Size/Space" actually mean.
This is, of course, information for how big the monsters are on a battlemap. For the actual physical size of a specific creature, you'd have to check their description. It might give you some info, or if not, you'll have to make a ruling on it (maybe by looking at something comparably sized that does have a listed size.)
The Monster Manual defines creature size as:
how much space a creature of a particular size controls in combat.
| Size | Space |
|---|---|
| Tiny | 2.5 × 2.5 ft |
| Small | 5 × 5 ft |
| Medium | 5 × 5 ft |
| Large | 10 × 10 ft |
| Huge | 15 × 15 ft |
| Gargantuan | 20 × 20 ft or larger |
A creature's Size category does not directly define its physical size, rather it defines the area that it controls in combat. A horse is size Large, but obviously horses are not 10 feet long by 10 feet wide. And on the other end of that, a Gargantuan creature has no upper bound to how big it can be - it may control far more than a 20-ft.-by-20-ft. area in combat.
The Size section of the basic rules contains this info as well.
