Hamster balls are not safe for several reasons. One when hedgehogs run around they WILL poop and pee. There is hardly any ventilation so the poor hedgehog will be breathing in their own poop and pee.. And with the rotation of the ball it will cause the hedgehog to be covered in it.
Also those little slits that were made to ventilate the ball are big enough to get a nail or toe trapped in it and the hedgehog can lose a toe.
No matter what size ball you chose it is not safe or fair to put a hedgehog in it.