My wife and I travel in a 26'9" travel trailer and it's fine for the two of us for even longer trips (4-7 weeks). However, if I was going to be full time in a travel trailer, I would want something around 30' with living room and bedroom slide. Add a teenage daughter, and I think you need at least this amount of room, even staying in warmer climates where you can spend time outside. If you are going to make the move, get a 3/4 ton truck and a larger trailer. When the daughter is out and on her own, you can downsize and will know better what your needs are at that point in time. You also have to think of storage space and, a teenage girl needs her space and privacy.
I hope everything goes well....enjoy!!