An 8inch wheel is best for all dwarfs. All of my hamsters use the
Trixie wooden wheels from Zooplus, with my 265g Syrian getting the 11-12inch/28cm one, and all my dwarfs past and present (ranging from 21g to 70g, current robo was 25g when I last weighed her) have used the 8inch/21cm wheel.
You can check out
the posts I made a while back about various pros/cons for certain wheels. The 'karlie wonderland wheel' I talk about there is the Trixie wheel I recommend/link above. It got rebranded a while back, and given a nice makeover (used to have wooden rungs on the wood surface; now is a flat cork surface).