I tend to agree with those that say 15 or 16 ft. might be more comfortable due to more floor space. A lot depends on how you camp currently. We carry a large rain fly and are outside for cooking, eating and hanging out. Traveling with kids the ability to pull in for the night, set up for the night by leveling camper and take off in the morning easily is nice. There is something to be said for the wow factor of 5 people and large dog in a 13 ft. provided it is not a PITA to do it.
One thing to consider, will you cook inside? Do you need base cabinet with sink, stove and fridge
inside the camper? Are you comfortable with replacing that with something shallower or shorter that would free floor space? Could you run water line to a hookup outside for dish washing etc. Saw a Trillium
with added outside sink that sink is what the owner used all weekend. With 5 people the small fridge
is not going to be as useful, maybe a taller fridge
and shallow storage and limited counter or shelf?
Of course at that point one is best off getting a fixer upper so one is not paying for nice appliances and cabinets that you would be removing to sell as used.
I guess I too find the idea of using a small space efficiently a thought provoking idea.