I'm sure not an expert, but from what I have gathered by reading the archives on sportster.org, it looks as though 140/90/16 is the widest you can go with a belt drive and 150/80/16 with chain drive. Even then I think I remember reading that you have to reverse some screws or file them down some. Of course that's leaving things pretty much stock.
Did you see the scooter in the thread titled 'Misguided' ? I think that's probably what we're all looking for.
The stock tire is very tight. The 140 my fit but at speed it expands and then what? If anyone goes bigger on a stock bike it better be a good quailty tire to minimize expansion. I myself would not try it.