I've consistently gotten the largest Mac laptop available over the last 15 years. That's been 17", 15", and 16", and it's a very comfortable size on my lap. (It sometimes gets a little cramped when a large cat wants to share my lap with it, but I can usually manage.)
Naturally, this is going to vary depending on your physical size. A person who's small and slight of build isn't going to be as easily able to use a 16" laptop as me.
The ergonomics of this also depends on the laptop itself. The large Macs are sort of uncommonly good at being used on the lap as a result of lacking the numpad and centering the keyboard/trackpad.
My experience with trying to use my 15 inch Dell w/numpad on my lap is a bit more miserable, because I either have to position it off-center and hanging off my right leg, or type with my hands off-center, which becomes really uncomfortable quickly.
I am tiny, though, so perhaps this would be less annoying to someone with bigger legs where it wouldn't be teetering off constantly.
Naturally, this is going to vary depending on your physical size. A person who's small and slight of build isn't going to be as easily able to use a 16" laptop as me.