what age does the girl look for?

Early to mid-teens is usually the average you see in anime. so 12-16ish

is it full body or head to waist?
thank you

That's up to you. I want the text to be most prominent, so the figure is window dressing. She can be full body alongside the text, just a bust beneath the text or above it. She can even just be a chibi character at one corner. I leave it entirely up to your imagination and what you think looks best.