I've met people who FT for over 7 years in a 18 ft Casita. Another mother and her adult disabled son FT for over 9 years in a Class B.
You can do it in whichever size rig you want. Just be sure that the rig meets your requirements, and you are willing to make the tradeoffs needed for a smaller rig.
For us two essential requirements were a bed slideout which would hold our Sleep-Number bed, and two recliners. Rainy days happen.
We also stay for a few months at a time while volunteering, so that was a consideration.
I don't have trouble finding camping spots, but I do occasionally have trouble with access to good camping spots.