I flew with my 13 Pro Max for the first time today. It is a little thicker (millimeters?) than the 12 Pro Max, but with an apple silicone case on, it still fits very well in the slot on the
DJI Mini 2 controller.
I didn’t do any running around, but I did hold it upside down without a second thought and it held as well as the 12 Pro Max. If anything it’s a little tighter fit which is a plus with my case.
I can imagine some monster protective cases making those millimeters more consequential though, so in terms of fit, ymmv.
In terms of screen size, the bigger the better imho. I’ve even flown with my 12.9 iPad Pro on a stationary tripod, plugged in to my controller with a long cable sticking out. I’m sure I looked ridiculous. If it wasn’t so cumbersome, that’s what I’d use everyday.