.25 is the weight, not the size. All airsoft BB's are 6mm diameter.
As for the quality, yes it matters, get some good quality BB's like BB Bastard, KSC, Madbull, etc. or you'll screw up your gun.
Higher weight BB's are better for more effective range and not being affected as much by the wind, blades of grass, etc. I find .20's almost useless outdoors.
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