I go small with my games I use 650,350. Most users prefer a larger resolution either 800,480 or the highest 960,640 on android. Performace wise you should be good at up to 800,480. After that I think you need to be real carefull with your code.
Run my game on your device, you will see how silky smooth it runs even though on some screens you hae 4 or 5 zombies and mad bullets flying around, no slowdown. Go check it out.
https://play.google.com/store/apps/deta ... ion.undead