It is up to you how much space you think you need. I know people full timing in 18-foot van conversions and the 17-foot model of the Casita. For both cases, their full time lifestyle is mobile.
I've met a man full-timing and work camping in a pop-up, 12-foot box. His lifestyle was not so mobile, rather seasonal moves between work locations.
Personally I would be more comfortable at 24-28 feet, for separation of living or working areas from sleeping areas. But that's old habit, I have friends of my generation (WW2 children) getting into the small house movement or moving into tiny urban apartments.
However, most of the full timers, and even the snowbirders, I know are living with 300 to 400 sq ft of space, e.g. 38-45 feet of motorcoach or 38+ in a fiver. Some of the snowbirders make their moves with an additional vehicle hauling a 24-30 foot utility trailer full of toys, or a substantial boat.
The space has to be adequate to your chosen lifestyle, or your lifestyle adjusted to the space.