Unquestionably the Class A type of motorhome and most people end up in the 40-45 foot range, though there definitely are those comfortable at 34 feet and up. While there are a few people who do live fulltime in a B, it's not what most people would consider for more than a weekend.
A 40 footer with slides still provides only about 350 sq ft of living space, and much of that is taken up with cabinets and furnishings, so it's not really big. Fulltimers also need storage, because they carry everything with them. Clothing for all seasons, hobby & sports gear, tools, yard furniture, your financial files, etc. No Class B or even a large C can provide that sort of space.
The only way to answer your question in a meaningful way is for you to visit many RVs and imagine yourself spending a day inside. Step into the shower, sit on the commode, pretend you are stowing your clothing, dishes, etc. away inside, and so on. It helps if beforehand you make up a rough list of things you would take with you, e.g. how much clothing, kitchen gear, etc. RVs that look cute and roomy often fail miserably when you start looking for a place to put a crock pot and electric fry pan, or you want to stow away your gold clubs or fishing rods. Even finding a spot for dirty laundry can be a challenge.