Is this Figure on a small or Medium Base <Looks medium to me>

Medium. You can check by comparing bases to either R2-D2 (Astro will be bigger) or by comparing to another medium sized fig, like a stormtrooper. But of course if you don't have an R2.. then its a bit tougher to do the comparison .

Seems like as good a place as any to ask...

Is there anything in the game at the moment that makes Small matter? Unless I missed something somewhere, I don't recall seeing any.

pretty much only comes up with swapping CE's where the two pieces swapped must have the same base size.

Of course.. Some small and mediums can't be swapped by CEs...
Cause they're droids!