Hard to say what size motor you need without knowing how bigger disk your planning. A tiny little one about a 100mm diameter or a massive 600mm diameter one.

I myself have built a spinner (check out my profile for a pic) and the biggest problem you will find is that there are very few events where you can run them because the arenas arent up to it OR people dont want to fight spinners. (To bloody right I dont want to fight spinners myself unless their is some substantial reason to do so). This is going to be even more of a problem if robot wars the TV series has as it appears demised.

If your still intreseted in building spinner despite the risk of hardly ever being able to use it Im only to happy to advise you. But hard to say more without knowing the sort of size of disk your thinking of. If you can tell us this Im sure there are plenty of people about who will be happy to advise you.

Richard Wenman
Team Mayhem