If building a traditional style teardrop (I've not built yet) I would be restricted enough on tow weight that I would probably limit my trailer length to 10 ft. With lighter methods I think 12-14 ft is not only perfectly doable but far preferable, because I have 3 large dogs.

Looking at what the basic teardrop is designed for, you need room for a bed and a kitchen. So 8 ft is okay for many and 10 ft is a really nice length with a generous amount of room. Would imagine that if you go too long there is the inconvenience of needing to crawl a little distance to get into bed rather than just sitting in the doorway & swinging your legs around. In the age of premade trailer frames, the simple fact that the most affordable are 4x6 or 4x8 probably helps continue the tradition. Going a foot or two longer without increasing width or adding more cabinetry wouldn't had too much weight, I would think.
If you want to build longer & have the towing capacity for a heavier trailer, you might consider using a pop-up tent trailer frame from a trashed, derelict or "hunter special" pop up trailer. Some have managed to get theirs free from someone wanting it haul off. Baring that you might try Craigslist. In our area there's usually at least one or two "needs TLC" pop-ups going for $300-400. I believe the "Road Foamie" is in the length range you want and is built on a pop-up frame. Here is the thread:
viewtopic.php?t=45698 There are likely others on TNTTT. (Can picture a couple but can't remember the names to search for.)
Edit: Just a note. If you build a raindrop or bow front style you could build out onto the V-area of the trailer tongue which adds usable dog sleeping space without increasing the overall length of the trailer.