HTOTM: FUSION
It's sort of self explanatory, you draw the head as a flat image. Use any size of paper you want scan it in, and crop out the actual head into a 128x128 sized document in any graphics software.
You can use any size, but you need to scan it in the computer after. With that being said, you can change the side how ever you want after.

I recommend anything as long it is the same number of inches/centimeters on all sides.
Try avoiding caps, it is helpful for other people.
Do you have any art programs you might want to use to fix your hand drawn image? For coloring or resizing? If not, I suggest using GIMP. You can google search it for the link is gimp.org. It is a free program. If you are wondering how to make heads here there are many other tutorials on how to.