A couple of things to think about. There is nothing wrong with keeping a trailer for 10 years or longer. Our last 2 trailers were 10 years old when we replaced them. They were still worth 50% of their original cost. Believe it or not smaller trailers are easier to sell than large ones.
The kids will have fun no matter what you decide. When they are adults they will not remember the size of your trailer(s) they will only remember how much fun they had and how much time mom & dad devoted to them. Bottom line is that only you can decide what is "too big".
We are friends with, and camped with 2 families who raised a lot of kids, one family had 5 kids, the other 3. Both had pop-up campers, as the kids grew older they put them in tents. Our daughter bought her own tent when she was in her teens. We had a 17' hybrid from '03 to '13. The kids from all 3 of our families are all adults now but still avid campers.
That said if you are really looking for something larger, but still within a reasonable length and are open minded consider a hybrid, especially one with a slide out.
