My best friend that lives 5 minutes from me has a 14x14ft tramp in his backyard. I've had a ton of time on it including with a practice board made of plywood, pipe insulation, and old bindings. It will help you a lot with your in air control of spins and inverts but setting an edge and starting the spin on the snow is not something that can be practiced on a tramp. I can easily spin 900's on a tramp but spinning a 360 consistently on the snow still escapes me. It drives me insane since I can 540 on flat ground on my rollerblades.
An 8x14ft tramp is a bad idea since you don't know which direction you may go when you start trying off axis stuff. I have flown into the net of my friend's 14x14 after trying a backflip 360.
2011 Smokin Buck Ferton 155
2013 Flow NX2-SE
2013 Flow Hylite boa
Burton Stagger pants/jacket