There's lots of good info in the Build a Jib 101 cult.
But it's all about personal preference. I'd have maybe 15ft in between the rail/in-run ramp to give enough room to set up. The ramp could be around 4-5ft tall depending on how much time you want to put into it and how much speed you want. There's no good formula, just basic guidelines. My advice would be to look at pictures of other people's setups and go off of that.