All the meanings of "size" is just like Avalanche's post, a little supplement:
the entity "origin" is not all the entity's center in many case, it is the entity's "eye-position" which tie to the entity's model.
for example, the "money-bag" model ( which use in my infinite money plugin) 's "origin" is at it's underside, that means it's "maxs" & "mins" size are not "symmetrical" parameter(z axis).