According to the manual for the 26C the length is 9’8” of 3/16” of 7 x 9 stainless with an eye at the upper end and an eye and shackle at the lower end. The manual does not specify whether the length includes the shackle. The shackle is intended to allow the boom to easily be disconnected and lowered to the deck.
If the length of your current hangers work well with your sail you should stick to that length. There are variations in the cut of the sails in the fleet so you want a length that works with your sail. My belief, not supported by any technical training, is that shortening the cables will increase the angle of the boom with the result that hardening the choker will put more tension on the leech of the sail and less on the foot. Conversely lengthening the hanger will result in more tension on the foot and less on the leech. I also suspect that small changes in the length will not produce any noticeable difference. If the boom hits the choker cheek block the rubber bumpers are likely to degrade more quickly. If you are a member of the INA you can consult with Mike Quill, who can let you know if I am correct and the range of lengths you have available without impacting the function of the choker.
My question to you is: What are the benefits you are hoping to achieve by going to Dyneema? UV penetrates and damages the surface fibres of Dyneema. The depth of penetration is about 1/8”. On 3/16” line that is effectively the entire thickness of the line. This means you can’t go smaller than 3/16” so you do not gain an aerodynamic boost and there will be little weight savings, may 1.5 pounds. Coated 7 x 9 stainless wire will long out live the Dyneema.
I would assume the length does not include the shackle.
I share Mark’s intuitions about the effects of shorter or longer hangers
Duration of dyneema under UV exposure is very sensitive to your latitude (since that heavily impacts actual UV exposure). My recollection is that Practical Sailor recommends giving uncovered dyneema 3 years in the Caribbean, but suggests it may well last up to ten in Northern Europe.
The safest choice if using line rather than stainless wire would be to use a dyneema-cored and polyester covered line, or planning on a regular inspection schedule.
I’ve also seen warnings that you should use closed thimbles with dyneema splices rather than the cheaper eyes meant for stainless wire. Those are basically a curved channel bent into shape with a gap between the ends. That gap can end up chafing the eye.
I’ve been working with dyneema for awhile, and like it for many things. I haven’t developed the skill and acquired the equpment yet to ensure a line of an exact length, although I can probably get within a few inches. Although dyneema doesn’t stretch after being loaded, there is a significant difference between the length of a 12-strand loose dyneema line and one that’s being pulled 'til it resists. If you think about its resemblance to a Chinese finger trap, this makes sense.