By looking at the picture of the back tire and fender, I don't think I would go over 23 inch. Anything over that and they might rub the fender.
BTW, have you asked this question over at the High Lifter forums? Those guys over there could probably tell you exactly what would fit and what wouldn't. Almost everybody you see over there has a lift and bigger tires.
looks like you would have to extend the swing arm to keep from rubbing if you wanted to go much bigger. also on the front, turn the wheel all the way to one side then see how much room you have from scrubbing on the frame and other things.