[Boyso]

.25s are good. Indoors, you can use .20 as the BB is not affected by wind, or you don't have to penetrate bushes and grass making the BB reach its target a tad faster. .25 packs a bit more punch though.

However, many players still use .25 for indoors and .28 & heavier for outdoors.

I guess it's really personnal preference. I use .25 in my sub-350 fps gun.