I picked 64x64 because that seems like a good middle ground between graphics quality, and the amount of work I have to put to draw something nice. (At 128x128, it'd be very painful.). But I then realized one caveat.
I wanted the size 480x480, but later realized it makes each pixels 7.5x7.5 squares, which isn't right. The only dimensions with multiple of 10 I can get is 320x320, too small, and 640x640, too big.
so I ended up making the size 512x512, which isn't a nice multiple of 10 but the size's ok.