I was thinking of increasing the rear diameter from 15 to 16 with slight increase in width from 180 to a bit more.. But seems like 180 is fine..There is only an OEM rear wheel for the VL800. There are no aftermarket options available. The width of the rear wheel is such that it will actually make the tire taller, rather than the desired wider profile. Because of the narrow width of the rim, the tire must be squeezed to fit, making the aspect ratio taller than it otherwise should be. Once it is on the bike, you gain a whole 3/4" in width over the stock tire.
There are no gains in performance, handling or aesthetics. It will however cause the sidewall to contact the fender bolts when you are heavily loaded and hit a bump. If your bike is lowered at all, it will also contact the fender liner as well.