With the slim one attached, it measures 77mm, with the large one attached it gets to 86mm. The difference is a little deceptive though, as the slim one only gets there because it has a little bulge at the back, while the larger one provides actual support for your ring finger.

I suppose the normal position to hold is to apply pressure to the top and right side, given that the mechanical buttons require very little pressure. I personally don't enjoy that type of grip though, I strongly prefer a spot where I can place my thumb when I'm not pressing buttons. That's why I chose to put 'blanks' (which are included, both as individual buttons and as 4-button rows) on buttons 5 and 6 (you can see and play around with the configuration here), making them a flat surface for my thumb instead.

I've also done a little experimenting on your behalf by placing small objects between the mouse and the largest of the two magnetic side-grips in order to add a few extra millimeters to the mouse's overall width. For me it remained quite comfortable to use, although if that's something you'd be considering, I'd recommend trying that yourself first. After all, just because my hand didn't notice the little gap that creates, doesn't mean yours won't.