its hard to tell, just be creative and have a good design before you start cutting wood. try building with what you have but If you run out of wood or don't think the wood you have will fit into your plans you can always go buy a few 2x4's, there only 4 or 5 bucks for a 12ft.
I havnt really decided yet but now that i think of it 3ft sounds decent. I can always go scavenge for some more wood. There are loads of new houses being built right around my neighbour hood and they throw out loads of 2x4's