It's going to depend on the amount of VRAM available, and supported API levels.
So the best solution would be to find numbers on the average amount of VRAM a computer has, and target that with your max.
Based off your numbers (rounding up, and simplifying):
256x512 * 16 * 4 = 8,388,608 bytes, or ~8 mebibytes. Unless you're targeting hardware from the 90s, you're fine.
(size) (#) (bytes per pixel)