If you are going to use the existing rear caliper you need a 280mm rotor. It is cheaper to find a front Nomad rotor,
http://cgi.ebay.com/ebaymotors/Kawasaki-VN1500-01-Vulcan-Nomad-Front-Brake-Rotors-L-R-/160576975897?pt=Motorcycles_Parts_Accessories&hash=item2563223419 , as you'll have $150- or so in cutting one down and grinding it. I am running a 170/60/17 on my Meanie rear with plenty of clearance. I can't imagine a 160/60/17 or a 160/70/17 giving you problems. I think the Meanie wheel is 5". Good luck as you will love the results. It's given me a much more fun gearing for in-town riding.