Wait a minute! Dave said WHISKER pole.... you asked for SPINNAKER pole... that's two different animals!
A whisker pole's length is determined by the
size of the sail....or sails... it will be used with. An adjustable length whisker pole is common but not mandatory. The foot length of the sail will often determine the optimal size.
A spinnaker poles length is determined by the
boat's dimensions. In most cases this is the "J" dimension, or foredeck length, of the vessel. Here are the
basic dimensions for your boat Hunter 40.5 the
"J" is 13.25 ft.
Besides extra rigging considerations, a spinnaker pole on a boat your size will have specialized end fittings predicated on the manner in which the sail will be managed. i.e. "end for end" gybe or "dip pole" gybe...
A whisker pole attaches to a sail's clew and is simply there to hold the sail out... it takes a far less load than a spinnaker pole... which attaches to the windward corner of a large symetrical sail and deals with a much higher degree of torsional load.... that's one reason you can have an adjustable whisker pole... but not an adjustable spinnaker pole....the internal mechanicals just wouldn't hold up.