Go check out fmx ramps . More specifically the rev 1, for the fact that this is what a lot of peoples ramps are based off of. If you search around on that site a bit it should help you out with all your questions. I had the exact same questions as you and this site seemed to help me out the most. Though the rev 1 is bigger than what you want, the site will still give you info on building your own ramp.
"Go Big or Go Home"
'09 Ktm 450 sx
Old quad- 06 LTR 450
Older Quad-01 426ex
-98 Camaro v6, previously supercharged
-69 Camaro soon to be restored
-53 F-100 soon to be restored
FQX, Mx, FMX, aka all purpose track in the works, built by yours truly