For me, the maximum would be a 25 footer.
I vowed I would never pull any thing longer.
Reasons: way too much hassle trying to get gas - Way too much hassle trying to park at a super market - Way to much hassle trying to navigate on a narrow one lane road in many of the state parks and lastly; take a look at some of the 12 and 25 foot overhangs behind the rear wheels and visualize how quickly things are going to be damaged.
With all that said: This past October I hauled our brand new 40 footer from the dealer to our permanent lot where it will stay forever and although I had no trouble, that solidified it; I'll never haul anything longer than 25 feet
Jack L