If you do not own a TT then rent one for a week or so on the road. You will learn, in short order, about what you like and do not like about pulling one of these barns down the road.
10 mpg, crosswind buffets, 250 mile days, 55 to 65 mph are the things you will notice more then the size of the campsites. You will learn about the abilities of your tow vehicle and a bit about what is the "right size" TT for you.
When your are done, you give it back and reflex on what you want and do not want in a TT. That is what we did.
Later we choose a 25 ft. TT with a bed you can exit without crawling over your spouse, a large bath, pantry and linen space, we gave up extra sleeping space because it is just the two of us. Our TT is small enough to wind through city streets to find shops etc. or pull into the back of a bank parking lot for a quick lunch.
The biggest drawback is also its greatest asset. It is so convenient you lose the feeling of camping. Of course this is not an issue if you do not like the sound of wind on canvas with critters scurrying around at night.
Twenty six foot 2010 Dutchmen Lite pulled with a 2011 EcoBoost F-150 4x4.
Just right for Grandpa, Grandma and the dog.