best tire size for mini tub?

Yes that is what I would recommend. You don't HAVE to shorten the rear end but it may limit your wheel choice. You'll need a lot of backspacing to keep the factory rear the way it is. My recommendation is to choose the wheels you want and make the rear work for that, whether it requires shortening of the axles and housing or not. I'm sure someone here with a Duster will chime in on what backspacing you'll need for that combo. I would guess somewhere around 6.5" backspacing or so, but I'm not sure of that.