In fact, out of all the sizing terms in golf, the word "Jumbo" is most often misrepresented. Some companies use the word "Oversize", which is the same thing as Jumbo. Here's a quick overview of all the approximate sizes (un-installed), to give you a starting reference:
- Standard: ~22mm outer diameter
- Midsize: ~23mm outer diameter
- Oversize: ~24mm outer diameter
Different brands have different specifications on each size. So there isn't an exact diameter for a given size. But in general, you can see there's approximately 1mm of difference between an uninstalled Standard and Midsize grip (and likewise between a Midsize and Oversize) as well.
When it comes to truly Jumbo/Oversize grips, there's one brand that far exceeds the typical outer diameter range, and that's JumboMax. True to its name, JumboMax grips are truly Jumbo. Even the JumboMax Small is over 16% larger than most Jumbo and Oversize grips. Here are the diameter sizes of the most popular JumboMax Putter grips.
JumboMax Wrap Small - 28.85mm diameter
JumboMax Wrap Medium - 29.24mm diameter
JumboMax Wrap Large - 29.87mm diameter
JumboMax Wrap XL - 30.84mm diameter
What's the real benefit of Jumbo grips?
So glad you asked! If you've ever written with a pencil or pen and felt your hand start to cramp up, then you understand the how a small grip can be painful. When gripping your club, if your grip is too small, you can similar feel the urge to "over-grip", and thereby strain and possibly cramp your hands. Oversize grip allow you to hold your grip lightly, which actually increases your control. When not "over-gripping", you'll have a better balance on the club, and be able to swing and putt naturally to the tempo of your body's swing.
Check out the new JumboMax Putter Grip too: