The funny thing is that when I decided I was gonna order the 13 Pro Max, my initial tought was that I wouldn't be able to use it one-handed anymore. But strangely enough I've become accustomed to it after a bit, even despite my pretty small hands. It's still a lot of maneuvering to use it properly, and reaching to specific parts of the screen is too much of an hassle. But most stuff works out fine one-handed.
I should mention that every time I use it one-handed, and I have to reach to something in the top without reachability (it's handy, but I find it tricky to use sometimes), I am so afraid to drop my phone. Again, I've got pretty small hands, and this phone is like YUGEEEE for me. And heavy, really heavy. So I prefer to use it with both hands, and only use it one-handed if there's really no other way. When I'm in public transit, I never use it one-handed, because of the constant sudden movements. But when walking, and especially when sitting, it's fine.
Nevertheless, I'm kinda amazed I'm able to use it that way at all. Never expected that when I switched to the Max from the regular one. Especially considering that even 12 Pro was quite tricky to use at very specific moments.