I also have long hands, 3/4" longer than your measured length, so I understand your problem. While it's kind of handy for me as a piano player, several years ago, I found myself feeling the onset of what felt like carpal tunnel pain in my wrist.. Several suggestions:
1. I use my mouse on a small folding "TV tray" next to my chair, and I elevate my wrist across a long bolster (sofa type cushion) about 20" long with a 3" x 5" in oval cross section as a support for my entire right forearm above the top of the mouse. This greatly reduces the fatigue and pain I would feel after extended sessions using my mouse.
To avoid getting the cushion drity with accumulated sweat, etc., I wrap it n an old towel, which I can wash wheneveer I do my laundry.
2, With this arrangement, I am comfortable using my full sized desktop mouse and my smaller portable laptop mouse. In addition to elevating and cushioning my wrist, I've found the most important factor to reduce fatigue is the feel of the switches when I click it. I feel greater fatigue if I use a mouse that requires more force to go over the click threshold,
I suggest going to a store with a good selection of makes and models and physically testing them to find one (or more) that feels comfortable to you.
3. Try this hand/wrist exercise when you just start to feel the onset of pain or cramping:
a. Hold your hand and wrist straight up.
b. Rotate your hand around your wrist in a helicopter motion. Do this with some force and speed so you can feel the streching of your wrist muscles. Then, reverse the direction of rotation, and repeat the exercise.
c. Sometimes, I grab my right wrist with my left hand and use my left thumb to put a little extra pressure over the wrist area in the first few inches below the heel of my hand while I'm doing this exercise.
I know this exercise helps, and I know elevating my wrist helps because I can feel the onset of my problem much sooner when I'm at another computer where I can't easilly arrange to elevate my wrist.
As for your concern about age setting in, I'm 73, and I've been doing this exercise for over 20 years, and it really helps.
Hope that helps.

Now that we've covered your problem, you may find this video amusing. :biggrin:
Rachmaninov Had Big Hands