Allowing width and height instead of size

[KarimAlaswad]

allowing for rectangular shapes instead of squares for precision would be great.

I think this could be done with margins, but I don't think margins show on the preview as empty spaces, idk if I'm missing somehing

[KarimAlaswad]

Interesting, so it could be done with the margin but the margin doesn't even work, it's start-margin and end-margin. so the default config is out of sync with the code, so this issue is a config/README update