In Canada, max. allowable length of a towable RV is 41' and max. combined length is 62' to 75' depending on Province. Personally, I wouldn't want to be much over 30'. If you get around much in the western states with all the twisty/narrow roads, hills/mountains, challenging grades and occasional sheer drop-offs, I'd imagine a longer & heavier TT would be a real handful not to mention tiring. Ferry fares in WA & BC could be a killer too.
Where triple towing is legal, seems the max legal o/a length is generally 65' but we met an owner in Utah towing a FW with boat behind it, 81' o/a. Looked crazy long to tow. Got back onto the interstate and doing 65 mph, he comes flying past us at what seemed like 100 mph (that's Utah!) and was out of sight in seconds. That was insane! Said he'd been doing it for years, even over into CA. Even with Utah being max. 65', it wasn't a problem for him.
Talking about length, wonder what it's like towing one of these truck trains in Australia? What if you missed a turn and had to back up, lol?