As far as we all know, size doesn't matter at all in combat - it's just a fun thing, like gender (so far - if they add breeding, then gender will matter). It does matter for the youngster (small rattata) and fisherman (big magikarp) badges, but again, those don't give you a combat bonus that I'm aware of.
I wouldn't worry about size at this point. If you have something extremely large or small, maybe save it as a novelty.
In the main games, size/weight can matter because it can affect how much damage is done by certain moves. The move Grass Knot, for example, does more damage on heavier opponents. But given the battle mechanics in Go, I don't see them being anything but aesthetics without a total game overhaul.