This is probably a stupid question but I am wanting to go to a wider rear tire on my 2000 Fxd Super Glide what is the widest I can go without having problems with the rear fender and rear belt.

 

Your stock rim will fit a 150 width. Some of the Dynas made that year had them stock.

 

OK what width of wheel and tire will fit if I decided to change my rear wheel and what is the widest rear wheel that will fit?

 

Check your side fender clearance, that will give you an idea how much wider the tire can be, unless you are willing to do more than just a simple tire/wheel swap.
Typically 10-20 mm is all you can do without other mods to keep from tearing things up.