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the biggest a Dremel can hold is 1/8 , you can try for something like this:
1/4 inch Large Head Screw Mandrel: Widget Supply
but I think 1/4" head is the biggest you can find for Dremel. If you can...

the biggest I have seen is 2" Diameter cotton sewn wheels and 1" felt wheels. Those work great for razors, I got them @ widget supplies a while ago.