Back a few years ago, I used anything > 80 in. sq. as my qualification for ULF status for an article I wrote on currently available ULF cameras or View Camera. In other words, anything with an film area greater than 8x10 met my definitions of ULF. That, admittedly arbitrary, definition excludes 5x12 and 7x11. My reasoning for this is that, logistically speaking, those two formats aren't significantly different to manage in the field in terms of weight and bulk (considering cameras, holders, lenses, tripods, etc.). In fact, in terms of bulk (and possibly weight), a 5x12 system (camera and holders) would take up less space in a pack/bag than a comparable 8x10 system. For 7x11, the weight/bulk issue is pretty much a wash compared to 8x10.
Anyway, that's the definition I've been using, but it's not meant to be exclusionary in any way. It's just where things, in my mind, start to go from big to REALLY big. Of course, shooters of 5x12 and 7x11 encounter similar product availability issues o those shooting the > 80 i. sq. formats. So, again it's a bit arbitrary.