Years ago, I read in a book or one of the photo mags about processing the occasional "mural" sized print. The technique was to build a frame from wood (1x4's, for instance) the size needed and then use a sheet of heavy plastic as a liner. It would work, but would be fragile. In more recent times I've seen this same technique used to build a basin for soaking wooden parts being stripped of paint on a remodeling site.
I've even read of people doing really large prints (I don't know where the paper comes from) by doing them on the floor with the chemicals in buckets and using a mop to "paint" the developer and fix. I don't think I'd want to try that one.
There are always alternatives. On a practical level, one is more limited by the sizes of paper available and the room to process it, rather than trays or chemicals.