By far the biggest limiting factor in size terms comes down to paper coating lines. There just do not exist any aqueous inkjet coating lines larger than 64". Even 64" is incredibly hard to come by (as you'll find with the extremely limited selection of papers that come with that extra 4". It's an insanely expensive venture to create a larger coater, and most papers actually come from the same few mills to begin with...
Otherwise, it would be relatively trivial to convert any larger "solvent" inkjet printer into aqueous-- they pretty much all use epson heads anyway. Just get a 77" or 104" Mimaki, Mutoh, or other printer and load it with aqueous inks instead of solvent based ones.
The only company I know of that produces a paper larger than 64" is Innova, and they only make an uncoated (like, ultrasmooth matte) paper which can be special ordered up to 96".
Otherwise, you're in vinyl world, hence the push towards reverse printing on plexi with UV curable....