How do I know which control cable length I need?

This depends on two things:

a) Are you doing a first-time control cable installation?

For outboards: Measure from control along an unobstructed cable routing to the center of the outboard. Then add four feet to allow for a loop that provides unrestricted engine movement. If it does not end on a foot length, then round up to the next foot and order that cable length.

For sterndrives and inboards: Measure from control along an unobstructed cable routing to shift and/or throttle connection. If that length does not end on a foot length, then round up to the next foot and order that cable length.