Best wheel for a Syrian?

[racinghamster]

I`m on the hunt today for a suitable wheel for my new inherited hamster Boris! He`s quite large syrian so I was thinking something around 8" inches diameter would be good? Or should I go a little bigger? I`m happy to purchase online today or tomorrow, so any links you guys can provide to good retailers who are fast at delivering, I would be much obliged and so would Boris!