Hamster wheel for Syrian hamster

[Mrweeniesowner]

Hi so I’ve been struggling to figure out which size wheel is perfect for my new Syrian cooper! I originally bought a 12 inch wheel and he could not even move it one bit so I bought an 8 inch since he is on the smaller side. I was wondering if this is considered to bend his back or no ? I have a picture for reference. I’m willing to upgrade I just wanted someone’s advice in order to fully tell if his back is arching or not