having a good deal of experience, make the rail no shorter than 30 feet, 12-8-12, 12-8-14, and 14-8-14, are all good lengths, anything longer is going to be incredibly difficult to move and work with. trust me.
the second tip i have is to make the rail decently mellow. if the kinks are too steep you need to put it in a really steep section of hill, thats also a bitch. the other thing with steep kink is that you'll miss a lot of the first down when you hop on urban. its no fun to land almost at the flat.