As far as I know size is just that. It doesn’t seem to effect much other than (it looks like) effecting the radius of things like bone spike nova and fire aura. I would also assume a potential downside is allowing for more tiny melee mobs to attack the golem at once, yet also create larger aoe on slams.
This is just with some visual guesstimation though. I got to where you have 10 minimum level on respecs and tested the fire golem node with a single golem, the double golem passive, the double golem with points in size to compensate, and a single golem with bonus size. In all cases I’m lead to believe size indirectly effects AOE size on certain golem skills.