I went for 14x17
I am personally moving to 14x17. "Moving" means "I just ordered a 14x17 camera with holders". So I am putting my wallet where my mouth is (oh gosh...)
The reason I picked 14x17 is five-fold:
1. Interestingly enough, in the ULF kingdom 14x17 is a standard. Why? Because of X-Ray film. In fact, there is plenty of X-ray orthocromatic film around. Granted, it is not T-Max 400 by a loooooong shot, but if the you-know-what hits the fan and ULF film disappears I can survive on ortho film. There is also some intriguing infrared film in the 820nm range. Nothing to brag about, just some tiny parachute.
2. The lenses I already have (G-Claron 355mm, Nikkor-C 450mm, Fuji-C 600mm) cover the format nicely.
3. I started my ULF quest completely sold on 12x20. But then my style evolved and I am no longer shooting solely landscapes. At this point I don't find 12x20 general purpose enough: wonderful for landscapes but somewhat less general purpose than 14x17, and a drag to shoot vertical.
4. 16x20 was just too much: new lenses, even more weight/bulk, hence money+++++, etc etc. and 11x14 was not big enough. I am shooting 8x10 already, if I make a jump with contact printing in mind it'd better be a JUMP.
5. My film holders will be able to take 12x16 as well, a popular size in Europe ('popular' is all relative in ULF, of course). Yet another fall-back if film becomes an issue to source: I have two options.
My one cent (after ordering the camera and the holders I do not even have two cents left)
P.S. Having said this, I am hanging on the 50 sheets of ADOX 100ISO film I was able to find and put in the freezer. With all the chit chat about Ilford runs, Kodak runs, this will happen and that will happen blah blah blah my bottom line TODAY is that this is the only 14x17 film I was able to find.
P.P.S. ULF is becoming a game for people with no nervous system, low blood pressure, or hopelessly optimistic.
Cheers