What sizes of Airsoft BBs are better than .20g?

.2g bb's are the lightest bb's anybody should be putting in their gun IMO. With an AEG .23's or .25's will give you lower fps but your accuracy will be distinctly better and your range will slightly increase. I would look up for some TSD .23's somewhere. See how those work out. If you feel that the bb is just going to slow, move down to .2g's. If you feel that your gun can put out some more power. Grab some .25's and see how those work.

As your power levels go up, so should the weight of your ammo... heavier ammo is more accurate... That UTG Warhawk is best with 0.2g though.

Better is heavier. The advantage to heavier ammo that it is more accurate, hit harder, and goes through brush better (good if you are playing in a woody area). However the drop the speed down by around 50 fps. There is an equation to know exactly how much it be better by but it is confusing. The other disadvantage is that they are harder to find in stores so you will most likely have to order them off of the internet which will raise the price. The only thing I use .12 gram bbs for is shooting it in my UTG M87 when I mulitshoot so that way it has a larger spread.