medicine balls

My 14 y/o daughter would like to start training with a medicine ball. However, they come in varying weights and I'm at a loss as to what size I should consider for her. She is 5'7" and weighs 130 (if that makes any difference. The soccer-size balls come in 2, 3, 4.5 and 6 lb. weights. Any FAST recommendations (lol -Christmas gift!) would be appreciated! Note: I don't want her to become the Incredible Hulk here so I'm not simply looking to buy the heaviest if it isn't going to benefit her.
Thank you!