Only you can figure out how much living space you need. I'm OK alone with the roughly 22x8 of my 30 foot C, it was OK for a couple weeks with two people, total madness with 5 or 6 for any more than a few days. I could do OK with a little less, have friends who go long term alone in 16 foot travel trailers, and have met couples full-timing in an A-frame pop-up on a 12-foot box. Others will consider nothing less than the 45-foot maximum for motorcoaches and want an additonal 100 sq ft in multiple slideouts. Obviously what works for me doesn't work for them.
You need to get into many different RVs of different sizes. Try out the space.
Best place to do that is a fairly large RV show. RV shows are put together by dealers or groups of dealers, the larger the group to more variety you'll see. Mostly the bring stock they are more pressed to sell, because at show buyers can be an easier sell. Sometimes, not often, they will bring used stock.
But don't worry about new vs used at the show, you are going there to figure out what fits you, not to buy. We visited three very large shows, a half dozen smaller local shows, and many dealers working on "what do we want," but when we got that sorted out it didn't take long to find what we wanted slightly used.
Keep in mind, full timing means you don't have a home base. That means you'll be carrying everything you own, or at least everything you think you'll need. That means a lot more space than needed for occasional use, or even for the kind of snowbirding where you have one place at each end, and might have stuff in storage at each place (like my cousin's 30 foot boat he leaves in Florida, uses a different one in Michigan).