The iPhone 12 Pro Max is the exact same width and less than 1/8" taller than your iPhone 8 plus, the same as my old 6s plus that I replaced with the 12 pro max.
This is what I don't understand: basically, the 12 Pro Max is the same size as the old "plus" phones. I honestly don't notice the difference at all. Do you really feel that the additional 1/8" height makes it hard to put in a pocket?
Anyway, it's just a personal thing and Apple certainly has a size to suit almost everyone now.
https://support.apple.com/kb/sp768?locale=en_US
https://www.apple.com/iphone-12-pro/specs/