I have a 14' and it does the job. I had a 12' before and it did the job too, but you better be perfect. Definately get the safety net, it has saved my life a bunch of times.
I emailed the guy from guru trampboards and he said I should either get a 10x17 rectangular trampoline or a round one bigger than 15 feet. I am most likely getting the rectangular one because it will fit better in the flat spot in my yard.