Allow different canvas size

[jkl3848]

Allow to select different aside from the default 32x32 such as:

• 16x16
• 64x64
• 128x128

This lets the user make larger sprites, and allows for making smaller ones without having to crop.

[Kully]

This is a really good idea. I might be able to make a PR for this myself in the coming days. Seems straightforward enough. :)