Ideally you want the same rolling circumference to keep the speedo/odometer close to stock.

Here is the formula for figuring your tire's diameter. Take the tread idth,(180)and divide by 25.4. Take that total, and multiply it by the tire height in percentage (0.70). Take that total and multiply by 2. Take that total and add it to wheel diameter(16) = 25.92"

Stock = Rear 25.45 .............. Front.....25.21

180/70/16 = 25.92
200/60/16 = 25.45

180/55/18 = 25.80....................130/60/18 = 24.14
200/55/18 = 26.66....................130/70/18 = 25.17
200/50/18 = 25.87....................120/90/18 = 26.50

I believe the stock front fender will also take a 21" diameter wheel and not look too bad.

You can do a 180 with only a little mod to the rear subframe and brake stay mount; a 200 with more 'drastic' mods.

Doc