The size varies depending on the folder in which the assets are located.
The ones in the '200x' folder serve as the base size.
In the 'XP/VXace' folder, the base size has been doubled.
For the 'MV,MZ' folder, the base size has been tripled.
As for the base size, most humans, animals, and monsters are drawn in 16x16 pixel units.
Some larger ones are 32x32 pixels.