When deciding what size half pipe you wish to build, consider things like space available, skill level, and how you want to skate your half pipe. I would like my half pipe to be some what a challenge and to allow for progression. I also don't want it to be a monster so one day my kids can learn on it. I'm really not limited to space since I live on a small farm but I would like to be skating sooner then later...I don't want to be building forever. You can always expand the width later. Taking all this in to consideration, I have to decided go with a 5 foot tall x 12 foot long half pipe.
I will need approximately 384sqft to build this ramp. This is with 4ft wide decks, which I may or may not decide to increase. I have also quickly decided on 12ft long of flat, this may also change. Increasing or decreasing these two variables will have an effect on the space needed, so I'll will need to lock them down before doing any layout.
HOW TO DESIGN A HALF PiPE
HOw TO BUILD a half pipe