I fly often and I usually choose the aisle because it's the only seat where my legs have a chance to fit, because I'm going to put them in the aisle.
If you are a tall person you may use this explanation:
I'm sorry but I chosen the aisle seat because I'm too tall to seat comfortably in the other seats.
This will clearly explain the other person that you are not refusing to switch seat because you don't like her, but because you have some real issue with other seats.
Obviously if the other person is way taller than you, give her the seat ;)