Hey, by 16x16 and 32x32 you mean document size?
I'm trying to draw a square isometric box by hand and it iseems like it's not possible to fit it in properly using 2px or 3px corner (pixels at the center of the box). You can draw a 4px corner and it will work.
This script doesn't support it, but you can generate a 3px corner box and then mirror it, to make it a 4px corner one.