I use the 2 mm for my light leaders like Kathy Scott's. 2 wt to 5 wt. But I DO use the 3 mm on my 4 to 6 wt and they are just fine. But like I said, I got 3 mm from Streamside and they were HUGE compared the the 3 mm from Niche. I think it is thickness also.
Check this chart out:
http://www.streamsideleaders.com/TIPPET%20RINGS.htm