So I suck, and it didn't occur to me to put the size in our bb entry. Still so busy I haven't had time to update the photos yet. 6mm:
.20
http://www.ascarmoury.com/product_in...roducts_id=461
.25
http://www.ascarmoury.com/product_in...roducts_id=462
.28
http://www.ascarmoury.com/product_in...roducts_id=463
If you're in Toronto, local pickup is always an option to save some dinero.
As for your question... Indoors I'd go for .20 if you're looking for a close, fast game. They aren't as accurate as heavier rounds, but you get faster time to target.
In general, I use .25 for most guns, but .30's in my MK23 (propane). Slow (relative to .20's, say), but very accurate. .28's are a good compromise between .25 and .30.