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.)