I'm not sure about Syrians, but I know a dwarf should have one at least 6.5'', so I'm guessing a Syrian needs a bit more. The only thing that can be harmful to them is if they have to keep running, repeatedly, with an arched back, and the more he has to arch, the worse his back will get. Preferrably you should have a wheel large enough that the animal can run almost flat, and won't need to bend. I hope that helps
