Length: narrower than the distance between bindings of the smallest board you expect to carry.
Angle: Good question!
Closer to horizontal makes it easier to get the boards out, and prevents interference problems between the tower and the bindings on the top board.
There is a significant advantage to having the boards more vertical, which is reduced width clearance. This is important when you tie up to other boats, especially like a houseboat or similar. It is a real drag to have a passing wake rock the ski boat and cram the boards into the side of the houseboat.