25 to 35 feet. No higher than 11'6" (for the "average" user), and no wider than 8 feet.
But, there is absolutely nothing wrong with having a TT, 5er, or MH in the 40 foot range either. It's really all a matter of adjusting your expectations.
If you really want to fit comfortable in almost any campground, campsite, then stay under 25 feet. Otherwise you've just got to be a bit more picky.
We started in a tent and the back end of pick-up truck, then a pop-up tent camper, then a 25 foot, 31 foot, and 35 foot travel trailer. Now a 41 foot 5er. We've been comfortable with each one over the years. It's all a matter of how you are willing to adapt.
