We just spent around 11 weeks and 10,000+miles in our 24 foot Class C - but there's just two of us and a dog. That's all the length we care to heat, cool, clean, gas up, drive, and park. Sometimes 24 feet is even too long for getting into the real out of the way intimate and scenic little camping spots all over the U.S..
I'd recommend going as short as you can possibly tolerate. As others have said, the layout is more important than the length. Believe it or not, Class C width is also important - for not only getting into many out of the way intimate and scenic little camping spots - but also for crossing the many bridges and traveling the narrow county roads with no shoulders in so much of the Eastern U.S..