Thanks for the links, I guess the length would depend on space between spindles, waiting for my cannon dale spindles to get here.
No, you need to design the distance between the pivoting of the a arms at the frame to the length of the rack. ie if the distance between the a arms is 14" at the frame then you need a 14" rack. For those yurfdog racks you need to know the width of the rack without the tie rods.
I have a drawing that might help explaining a simple setup for a arms and rack. This drawing is for a 8" rack. The rack is drawn in light.