I'm the opposite; I tend to draw my heads too small. But I think I've improved on that.

But if I do draw them too big or too small, and don't feel like erasing it and drawing it again, I resize it digitally (and for me, I use Paint Tool SAI. Any graphics program should work.)

Thanks. As long as you can see what's wrong with the character then you are on the road to improvement. I have photoshop cs5. I think I can figure out how to do the resizing with it.