Not sure I agree with '24 or bigger' for a couple.
My wife and I traveled for years in a 189 single slide with no problems. We also had a 120# Akita with us. Admittedly, you had to step around the dog, but other than that, the 189 suited us well. But we get along well. Had we needed an extra room for one of us to cool down in, that room would have been outside!
But I recognize folks are different. I personally don't like towing something out of Lucy's Long Long Trailer movie. *(we have a 24 now for fulltiming for the extra storage).