For me, 16-18 feet might be enough, it would give me a bathroom, modest storage, a permanent sleeping area, and a work area and kitchen. I don't need a lounge area. But storage for the stuff you want with you is the big issue with full timing, and I would be covered by using a full-size van as the tow vehicle for a small TT.
But tight now, for seasonal/snowbirding, I'm hanging on to the 29-foot motorhome I've already paid for, as it has space to rattle around in (we've done trips with six people) and can carry about 2000 pounds of stuff. A 24-26 foot TT might have equivalent space, but it is not so easy to find that carrying capacity, some of what you think your life needs might have to go on the tow vehicle.
I've met folks who full time solo in van conversions, pop-up trailers on 10 or 12 foot boxes, 24-foot travel trailers (with 2-4 children). I've met couples and even singles who need 40-45 feet of motorcoach, or 38-40 feet of fifth wheel trailer. Much of this depends on how much stuff you need to carry with you, as a single human being needs very little space for survival or modest comfort.