Im sorry if this seems a stupid question but I just purchased a 05 CBR 1000 Repsol and was wondering if and how much I can increase the rear tire size on the stock wheel. Thanks very much.

 

The biggest you could and should fit is a 190/55 from the stock 190/50. Its a taller and more pointy profile. There are tons of threads about going with a 55 rear on this and other sites so should be lots of info to help you determine if you want to do it.