Hey, I just tried that out. Turns out that you can, just you need to keep the aspect ration the same (ie 256x256 becomes 512x512. They need to be the same for X and Y.
So, it's possible, but as Aeries said, you need to keep in mind the people you're designing it for. I'd suggest 512x512 (already 4 times bigger than usual NFX2 ones, yet not too big)