Aye, so working in 256x128 then scaling to 128x128 will result in pretty much the correct proportions. Of course there's the problem of losing detail doing that.
Normally I make a layer with a rough sketch of the head, then I scale it to the correct size. That gets used as a guide and I draw over it. Everyone has their own method, but in my experience unless your working with vectors I advise against drawing then scaling.