There's lots (and lots) of threads in here with similar questions, so if you just spend an hour or two clicking on the posts in Reader's Ramps, you'll find everything you need to know.
But to be a bit more welcoming

... yeah, you'll get a ramp into those dimensions. 20 ft is tight, lengthwise, but can be done. I've built a serviceable ramp in a 13x6ft space, so you can definitely manage to get one into 20ft.
This is where you need to go though:
http://www.xtremeskater.com/ramp-plans/ramp-tool/
Doesn´t like decks with dolphins or Bob Marley painted on.