What is the biggest diameter pipe you can put on a sportster?

I found some cycle shack 4" oval shaped slip on's but am wondering if that is a misprint?

Does anyone have them? I found a video on youtube but unfortunately it has no sound of the pipes just showing off his bike.

 

Head pipe is more important than the slip on diameter.
1 3/4" is best, 2" you will lose bottom end (torque)
4" muffler may look funny, and possibly get in the way.