it's pretty good

if you're looking for a general tut on drawing faces, found
this one with a quick google.
basic rules are pretty simple. the eyes are at the exact half-way point on the head. draw a line through the middle of the head and that's where the eyes are. the bottom of the nose is half-way between the eyes and chin. and the mouth is about 1/3 the distance between the nose and chin. these rules should probably be kept regardless of style just to make it look like a face.
regarding eye-size, that's completely a style thing. if you're going for realism, a good way to think of it is that you should be able to fit five eyes across on a human face. -o-o-. if you're using a cartoon style, the eyes are slightly larger (especially on villians), and if you're doing anime, even larger.
the shading could use a little refinement, but hell, i've done worse.