Not knowing your boat, I think I can say confidently, that the cable length should be just long enough to allow full extension of the board. Longer and you experience extras drag as well as undue movement and chafing of the cable against the trunk and points of attachment to both the CB and winch. I have to replace mine. I'll overestimate the length by about a foot. I'll attach the upper end to the CB winch and feed the cable down into the trunk. Then, attach the lower end to the board in the down position while Dragonfly is hanging in her lift frame. Make sure it's tight so the cable is taking about 5lbs (complete guesstimate) and cut off the extra.
If your setup is like mine, the point of attachment is well below the bottom of the hull when the CB is down. Measure the length of the board, from that point, to the top, add the height of the trunk plus the horizontal distance from pivot point to winch. Any length that needs to be wrapped around a pulley before attachment point should also be added in. For mine, it connects directly. This should give you plenty to work with, without wasting too much.
If you are trying to order blind because your boat is in storage and you can't measure. You should be able to find a good line drawing to scale online. Over estimate from there by 2' and you shouldn't be spending more than a few dollars extra. On the other hand, if you can't get to the boat you can't do the work either so you might as well wait to order your cable.
- Will (Dragonfly)