I have a 2007 fatboy with 21 spoke wheel up front n the stock 200mm 17 wheel in the back I want to put a spoke wheel in the rear with a white wall. What size wheel should I go with 16,17,18

 

IMO you should stay with the same diameter as the original wheel because of the gear ratio difference with the larger or smaller wheel your speedometer would need to be recalibrated and your final gear ratio would be off, either to tall or not tall enough.

 

18 it makes a huge difference in looks. You will need a custom white wall though