The biggest portable camera I've used is my current 12x15 inch Gandolfi, but I'm going to sell it, just as I did my 11x14 Imperial (made to my design in India some 20-25 years ago -- I sold six or eight of them, including two to the late Terence Donovan).
Increasingly I am convinced that unless you are into big prints via alternative processes -- and I am not immune to the charms of such prints -- you might as well enlarge 5x7 inch. After all, 3x is 15x21 inch and the quality should be indistinguishable from a contact print. If you do like alternative processes, well, I like 5x7 inch Argyrotypes and POP too.
ULF cameras are huge and heavy; processing is altogether too much like hard work; and running costs are enormous. I can see why some people do it, and admire their work; but I have also met those who would be rotten photographers regardless of format, and who would have done well to learn to walk (perhaps with 35mm) before they started to run with ULF.
Non-portable? Can't remember. We had a copy camera (on rails) at Plough Photography and it may have been as much as 24 x 30 inch but as no one ever used it above 8x10 inch (via serious reducing backs) this didn't mean a lot.
Cheers,
R.