I've heard of a number of people using them on 12X20 and 11x14, but a lot depends on the quality you need in the corners of the field. Since
ULF shooter tend to contact print, this does not became anywhere near an issue as in enlargement. Most users of such lenses stop them
down quite a ways. I just shoot 8x10, and know that I could limit out a 355 G even on that format in certain situations, like those involving a lot
of rise or tilt (not hard to encounter when you're trying to shoot redwoods from down in a creekbed!). G-Clarons are generally a bargain compared to other options, and work well all the way from closeup to infinity.