25rr. I had originally planned on the 19rr, then saw the 21rr which equaled more space. 25rr had another 4 ft. No brainer there and it was 1/2 ton towable according to my truck specs (Ford F150 with towing package).
What difference does another 4ft make? Well, the back bench seats make into a bed that is very nearly the dimensions of a california king bed. With a memory foam top it's pretty comfy. Also, if I had two motorcycles it will be easier to fit them into the 25rr. I haven't ever been in either of the other two just looked at pictures online. But, I mean, I'd hate to lose 4ft of living space.
Forgot to add, it's a pain to get backed into my driveway and it takes up the whole length pretty much. But, shrug, you find ways to deal with your problems.