We built a 12' double barrel rail, roughly 20" tall. Our drop in ramp initially consisted of a rectangular tower with an 8' piece of 3/4" ply (supported by 2x4s) acting as the tranny. The tower was roughly 6' tall and unequivocally too short! At those dimensions, it was impossible to slide the entire rail despite every combination imaginable of slippery stuff and water. We added another 2' to the tower and extended the tranny (a second piece of ply) in order to maintain the same angle drop in. This ended up working great.
Recap: A 12', 20" tall rail required an 8' tall (10-11' long) drop in.