Size
From The DarkMod Wiki
The size spawnarg is used to setup a default collision model for entities when they do not have a CM in either the model file or a separate .cm file.
This spawnarg is only used when the spawnarg noclipmodel is set to zero, and when mins and maxs are not defined.
Making a dedicated collision model should always be prefered over using this method.
See noclipmodel for further information and examples.