Forespar's recommendations...
...are at the link below. The used H26 I bought last year came with an ADJ 6-12, which Forespar says is undersized for my boat. They're probably right; the pole's friction twist adjustment has allowed it to collapse down to 6 feet under sail a few times, but no harm was done and I still use it. If you can live with a fixed length, you can save some money by buying the end fittings from Forespar and getting a length of rigid aluminum tube from a local metal shop.