38'to 40'are the most popular coaches. And now with big slides or some have up to four slides it saves you the trouble of driving a big long cumbersome coach that does not fit on most state or national parks. With big slides or four slides it gives you lots of square feet.
I myself was on the market for a 45'coach going full time. Good thing I listened to my friends that had been RVing longer then me. Kept my house and use the coach six months out of the year. Works great!