From Quake Wiki
void setsize(entity e, vector mins, vector maxs)
Sets the size of the entity's bounding box, relative to the entity origin. The size box is rotated by the current angle of the entity.
e- The entity who's model is being set
mins- The coordinates of the minimum corner of the bounding box (ex: VEC_HULL2_MIN)
maxs- The coordinates of the maximum corner of the bounding box (must be larger than mins) (ex: VEC_HULL2_MIN)