Oh! I love it!
I think I know a way for you to fix her head size. It isn't her head size, rather her hair is a bit bulky near the crown area. It gives a bit of a light bulb shape to her head from this.
I do love the colors and the simplicity of the background. It makes it look a bit like a comic book cover. Given that the character is from a comic, more power to you.
However, compliments aside, I'm not really fond of the blurry coloring when viewed at full size. I thought I was becoming farsighted for a moment. This might be because you're coloring with airbrushing or you're smudging to get the color and tones right. Try working with sharper and crisper brushes and experiment with different opacity settings for the brushes to get unique fades and color transitions. (Though, this might just be a bit of a personal pet peeve as the composition looks lovely from a re-sized view. )