If you look at your Renthal grips, they'll say just how thick they are. It'll be V29 or V32 on the thumb side.
As been mentioned, the difference is the outer diameter. You want thicker grips go with 32, narrow, go with 29.
IMO, 29mm is the way to go. I have 32mm Dual Compounds on my R1 now cause a local shop ****ed up my order and I don't really like them. Didn't want to go through the hassle of return/re-order, so I stuck with them.
On my R6 I have a set of 29mm Kevlars that I've had laying around for several months. Online vendor sent me the wrong Dual Compounds so they let me keep them and sent me the right set.
Seems I can't get any luck ordering these things. lol. Personally, I like the regular Dual Compounds better than the Kevlars, but to each his own.