In terms of length, if we were full-timing, we'd probably want a longer trailer, for storage space as much as anything, but we have happily camped for 6 weeks at a stretch in a 19-footer. Part of the reason for not going longer is that we do like to camp in more remote places in the western states and BC that either won't accommodate longer trailers, or where there are sites for longer trailers, but only a few of them per campground, so these tend to fill up first.
Any more, a lot of public campgrounds have a reservation system where you can go on-line, and see what types of sites/ RV length limits are available. Most RV parks advertise that they accommodate big rigs. If you've got your dream destinations worked out, it might be worth spending a little time on the Internet to see what trailer lengths they can handle.
I just checked the two major CGs in Glacier NP in Montana, for example, and found that they have 35' sites, but most were 27' or 24' limits. Natural Bridges NM in Utah has a 26' limit. Bryce Canyon NP "discourages" rigs more than 45' in total length in its Sunset CG. Priest Lake, a popular and beautiful state park in northern Idaho, theoretically accommodates longer rigs at Indian Creek, but the typical narrow, sharp right-angle back-in sites, are tricky for any size unit.
Of course, maybe you don't plan to visit these places, or would rather stay in a nearby RV park with full hook-ups if you do. So a lot depends on where you hope to camp, and your preferred type of destination.