Make the head in 512x512 if you want the best quality.
This method takes a small loss if you resize.
Just lay out the mapping and resize, colour and shade it in 512x512.
About the design:
Mapping looks decent, nothing special.
The mouth looks a bit low and the face looks a bit small.
Mapping of the back looks pretty boring right now.
Top of the head could be interesting.
I hope this will help you out.