My .02:
I'd get the largest trailer that you can:
A. Afford to Get
B. Are Able to Store

If you get a 16 footer, you can haul a good sized load to the yard, and if later you move up to scrapping cars or large machinery, you could move up to doing so with a 16 footer. I personally can't store anything longer than a 16 footer at my house, I barely fit my truck and trailer in my driveway.
Aaron