The front pockets on my jeans are 8x7 inches, or 7x7 if you want some margin to keep things from poking out. Anything up to about 4 inches wide is perfectly comfortable when I'm sitting, so let's say 7x4 for a device, minus a bit for thicker devices. A switch without the joy-cons is a good example of something that's right about at the limit.

An iPhone XSMax is 6.2x3 so it fits very easily. It's only a quarter inch larger than my own phone. An iPad on the other hand is 9.4x6.6, way too big. An iPad mini is 8x5.3, which lets it be crammed into the pocket but not be comfortable while sitting.