As long as you are not racing 15' would suffice over 18'. What the pole length does is to push the crew out enough so that the sail has as close to its foil shape as possible. So given that, the length should be close to the foot length of the sail. It's also desirable to be able to collapse the pole so you can swing it through aft of the forestay for jibes. They are expensive. They are also very useful for downwind sailing. If you scour the local boat yards you may find a mast, boom, or pole that could be cut to length. It doesn't have to be absolutely stiff so you can keep it on the lighter side. Before someone in our fleet gave me one, I used a telescoping boat pole. Certainly not ideal but better than nothing.