The next power of two value after 512x512 is 1024x1024 after that 2048x2048 etc..
Anything over 512x512 is gonna (just check in debug mode, notice how much cpu is beeing used and what is taking up the most memory) .
you can also do stuff like 512x256 and such if it helps maintain , and if its still looking distorted change your game to pixel rounding: ON & sampling to Point