It all depends, we have a 12 foot round one here, but use a rectangle school type trampoline for training with the snowboard school...
We also use Bounce Boards for training at the snowboard school, but for weight and size, we have a board with pipe lagging around the edge which is tapped on, that helps with the weight etc and awareness of size....
Google is your friend in terms of asking questions, and seeing what is available...
Height 5"11, Weight 200-210lbs, Born 1969
Regular Stance, Ability NOT GOOD
2012/13 114 days
2013/14 102 days
2014/15 57 days and counting
K2 Fastplant 154, Ride DH2 158, Endeavour Vice 155, Signal OG 155, NS Evo 157 (Stolen), Flow Quantum 155