What size medicine ball should i get?

I'm planning on getting a medicine ball for my workouts. I'm trying to build some muscle, tighten core, lose about 10 pounds of belly fat, and get abs.
I have this Nike training app on my iphone and it has great exercises with medicine ball.
but what size ball should i buy? I watched a video where a boxing coach explained 15 pounds is too much unless you do hardcore workouts and are a fairly big person. So i guess 15 pounds is out of the options.
I'm 5'5, 125 pounds, male, 20 years old. Should i get the 12 pound ball or 8-10 pound ball?

I'd get the 8 lb one. It's easier to do more reps than get worn out sooner by heavier. I'm no expert at exercise but that's my logic.