"If your trailer is so small that you have to go outside to change your mind, it's too small."
After twelve years of camping (for 60 to 90 nights a year), we are still very happy in a trailer with a box 139 inches long (less than 12 feet!).
The trick is that DW is very small, very agile, and very easy to get along with. ;)
All kidding aside, we are not looking forward to the time when we have to shop for a new trailer -- there is almost nothing on the market that is as small as we would like. We do a lot of off-pavement boondocking, and a little trailer can fit into the darndest places.
Oh, well. Maybe I will just cut one of those big ones in half??