A witch comes and offers you to change your height anywhere from 4'0 to 9'0

However you have to pick something in your body that will be below average or above average compared to other people in that same height. It could be your legs, arms, etc. You get the idea

YOU CANNOT PICK YOUR PRIVATE PART.

Everything else will be average sized based on the height you pick.

What height do you pick and what body part do you choose to be small or large for your height?