Just measured, 135mm to go from edge to edge on my main monitor, so a little less than I remembered. The mouse pad I have is 450mm wide. It takes me a little over half that to cross both monitors at this desk, which is a 24" 1920x1200 and a 27" 2560x1440.
I’m guessing you’re almost entirely using your wrist, then, with your forearm stationary?
I went ahead and picked up another trackball. I was surprised how quickly my hand went back to being proficient with it.
My wrist is pretty much locked (I have arthritis in it from an… errr… ummm rollerblade accident when I was in my 40s) so there’s limited movement - if I had to describe the movement it’s more from the shoulder, (which has arthritis from a unicycle accident when I was in my 50’s 
).
Most of my mouse control is actually from the tips of my fingers - the MagicMouse is apparently terrible ergonomically - I have not found that, but it might be because I’m so twisted out of shape!
Yeah, I guess at 40mm you’d be able to do most if it with your fingers alone.
I dunno, it’s definitely not for me. I’ll stick with my apparently unusually low sensitivity 
If you have a “claw grip” I guess it’s not that terrible
It should even help with teh tactile aspect of it…
Some news about my diverse experiments:
So far, the most successful solution was the 3D printed “pinky rest” for the MX master.
I even printed another one for my M705
