I would go for a longer style that you can still wear down and it won't look funny. Also make sure you pick something that will look good with your face shape. I have been waiting since 7th grade to get a mohawk and I'm 20 now. I'm just waiting for my hair to grow out long enough because I don't want mine to be too short either. Here's the style I'm going for:
except Hers gets shorter towards the back-bottom of her mohawk and I'm keeping my the same length. She even has bangs which is a goodway to make it look more "girl-like" I guess you could say. I'm also thinking of this one:
It looks like it would be easier to spike and the sides are a bit longer.
I hope I helped you out a bit. Good luck with getting your mohawk!