I have an odd-sized bed as well, @Mary4578 . It's 4 ft x 6 ft. I use full/double protectors and sheets for it and just tuck the extra material under the mattress as neatly as I can. It looks good, works well, and no one has complained.
Will the guests be making up your fold-out couch themselves, though? I suppose in that case they might message you saying the sheets don't fit. Best if you can determine when guests will need the couch and fold it out and make it up yourself if that's possible. Otherwise just explain about the odd size.
All the best with hosting!