More range than a floor crunch, more work for your abs.
Let your abs travel through the full range over the ball. Don't just nod your head forward.
Curving your lower back over the ball lets your abs start each rep from a stretched, extended position that a flat floor can never offer, so the crunch travels through far more range. The ball also shifts slightly beneath you, adding a stabilising demand to every rep. That stretch-to-squeeze range is why it stays useful long after floor crunches have gone easy.
One that puts your knees at roughly right angles when you sit on it. Too small and you are basically on the floor, too big and you will fight balance more than you train abs.
Flat on the floor around shoulder-width for a stable base. Bringing them closer together makes the ball wobblier and the exercise harder, so adjust your stance to set difficulty.
The ball adds a genuine stretch at the bottom that the floor physically cannot, which means more range under tension. Floor crunches still work, the ball version just gives each rep more room to earn something.
