Max size on the iGen 5 is 14.33" x 26" (Not sure about the previous models - I think the iGen 4 was the first extended sheet size model?)
You'd also need the thick material add-on to print anything over 350gsm (although I've gotten away with heavier on ours). I've heard this add-on isn't worth the money - it's clunky and doesn't run great from what I've heard.
Without having any idea on the volume, I'd think with the heavy weight and oversize requirements a wide format machine would be the best bet.
Max size for the next few weeks, sure.