The standing martingale is not meant to hold your horse's head down. It should be slack when your horse's head is carried in a relaxed position, and only come into play when the head is raised too high.
Judges are intelligent people, and if you are using a martingale to hold your horse's head down, they will notice what you are doing and penalize you accordingly. Using an overly short martingale will therefore do nothing but restrict your horse's freedom while jumping and make it more difficult for him to balance himself at other times.
A martingale that is too long is a far less serious fault, though not without consequences. It is less likely to come into play when you need it, and might even distract your horse by swinging around. If it is very long, I would also be worried about potentially catching a leg in it while jumping.
To determine whether your martingale is the correct length, attach it while your horse is standing relaxed. Lift the strap (the one that runs from the cavesson to the chest) up as far as it will go. If it...
...doesn't lift up at all, it's far too tight.
...lifts up to the throatlatch, it should be just right. When you let go of the strap, there should be some slack in it with the horse standing comfortably.
...lifts up further than the throatlatch, it's probably too long.