A smaller trailer is less livable. When you get to where the bed and couch have to be the same device (below about 25' or Overlander size ), you spend a lot more time shuffling the living quarters.
There is a reason why the Sovereign
size (30') is (was?) so popular and that is because it provides 'elbow' room for living without an excess penalty in weight or in parking restriction. Yes the extra ton and few feet in length can make a difference but only in a very few circumstances. With modern tow vehicles the weight isn't really an issue and those circumstances can often be handled in other ways.
The less than 20' realm is OK if all you are doing is weekends or if you have resigned yourself to rather severe lebensraum constraints (like many boat owners).
The Ambassador size (29') has a bit more closet and a bit more bath space than the Overlander. It is always interesting to see where Airstream puts the extra foot or two between models and where the big jumps are.
It should also be noted that the wider body that came out in the 90's is like adding another foot or two to the length. That wider body can also limit where you can go (some of our rallies have required a bit of road clearing to avoid Nevada Pinstriping, for instance).
The caveat is to watch out for the attraction of "cute" and focus on what you really want your trailer to do for your camping and portable living experience.