Been asked many times before and opinions differ. It depends too on the size of the sail. BUT nearly all agree it should be longer than a spinnaker pole, which is correctly sized as the distance between mast and forestay at deck height. Two popular opinions are that:
(i) it should be the same length as the length of the foot of the genoa;
(ii) it should be about 25% longer than the distance between mast and forestay at deck height. This seems a good compromise if you have more than one headsail.
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