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9's dependant on the offset are about the widest. I have seen 10's but they will poke too much. 9's will poke a bit. The countryman can take a much bigger wheel like 20" because of the clearance.
With trimming there rear inside fenders and the frame/metal area in the rear wheel well area I was able to fit 17x8.5" wheels and 245/45-17 tires. I'm also running -2.2 camber up front and -2 camber in the rear. Lowered on custom BC Racing coilovers.