What I'd the biggest tire I can put on my nightster I love the way they look

 

As big as you want, just depends how much work you wanna do and your budget.

 

Some have fit a 160 Metzeler rear tire on the stock setup. Do a search and you will find threads on this. Unless you have longer shocks than the stock Nightster shocks, then you might have issues with the tire hitting the wires on the underside of the fender. If you have $$ to spend then there are a few companies out there that make wide tire kits but you will have to replace the swingarm, fender and rear wheel to make it all work.