Trailer length doesn't matter IMHO. I think Westend was providing a guideline for common "lightweight" TTs that will be in that weight range. If you find a TT that keeps you under all your tow vehicle ratings, the length doesn't matter. A "25' TT" can weight 3500 pounds to 10000 pounds depending on materials used and what and owner puts in it. A 35' TT can weigh 6000-14000 pounds. A 25' 7000 pound trailer will tow just as well as a 35' 7000 pound trailer if both keep the TV under all its ratings and a WDH is properly adjusted.