For regular prints from 35mm film, I use 13x18cm (5x7") and 30x40cm (12x16") for the very best ones.
The biggest one was a 50x50cm (20x20") from medium format film, but that's as far as I can go with 40x50cm trays. Moving the print evently all the time wasn't exactly easy, but in the end, it came out ok. I don't find this kind of size neccesary unless you actually present your work in a big gallery. What good is a 24x36" print when you put it in your living room and view it from from an arm's length away. I prefer a well-made 5x7" or 8x10" that you can hold in your hand and view up close.