A standard spinnaker pole is as long as the J measurement on your boat – that is, the distance from the base of the mast to the forestay chainplate. This is purely a racing rule requirement; longer poles are penalized under PHRF rules. If you’re cruising, your whisker pole can be any length you like.
On a couple of occasions when I’ve wanted to run downwind with poled-out headsails instead of a spinnakers, I’ve found J-length spinnaker (whisker) poles to be too short to get the tack of a 140 or 150 percent genoa out far enough. I don’t like the sail to be too full; it tends to flap around as the boat rolls, which it will if you’re running in any kind of a seaway. I usually roll up the genoa until it sets fairly flat – I would do this in any case in strong winds -- but that means I lose valuable sail area if the wind is light.
Some cruisers carry an over-length whisker pole – about 1.25 times J – to use in light airs, but unless your boat is big enough to carry two poles easily (not a bad idea if you can get away with it), that adds deck clutter you could do without. This is where a telescoping pole, slightly longer than J, comes in handy. The pole will extend half its length again, which is extremely useful in light airs. I’ve winged out an asymmetric spinnaker with a fully extended telescoping pole attached to its clew, and it’s amazing how much more power you get. Only do this in very light wind, though, because a fully extended pole can’t take too much punishment. Forespar does not recommend that you use its telescoping poles with a spinnaker, as they aren’t designed to handle the tack loadings; Selden says it dpends on the pole section, boat displacement and other factors. Bottom line: if you destroy your expensive telescoping pole while using it with a spinnaker, don’t count on any sympathy from the manufacturer. Even if you’re just using the pole with a headsail, if it comes on to blow you’ll have to reef the pole along with the sail.