A small trailer in Washignton State, the 13.5k is probably fine.
If you plan to spend a lot of trips in really hot conditions, I would bump up to the 15k unit for very little extra cost to get a bit more cooling. If it's 100 degrees out, even with a small trailer, it may have difficulty keeping it cool inside.
Mostly done by rule of thumb. To apply a formula takes a lot of variables, how hot outside, how humid, how cold do you want it inside, how good is the insualtion, how often does the door open, is it in direct sun or shade, etc...