12' works... 14' is better... I LOVE my 15'er.
Wifey made me get the net, but I actually don't mind it most of the time. I get a little crazier and it has saved me, but I had no net for years and never took any falls I didn't walk away from.
Next year I'm sinking it into the yard. May have to go a little smaller when I do. No way I'd go lower than 12' though... and might even spring for the rectangular one. They seem to be WAY bouncier.