Chestnuts, and the ergots (located on the rear of the fetlock), are actually thought to be evolutionary remnants left over from when horses were the size of dogs, and had multiple padded toes. Most horses do have chestnuts on all four legs, with the ones on the forelegs above the knee, and the ones on the hind legs below the hock.
I am not sure if when you speak of "long" you mean from top to bottom, or how far they stick out from the leg. There will be some normal variation in length from top to bottom, and it is nothing to worry about. Chestnuts are not a blemish, and the size of the chestnuts would have no impact on the horse's ability to work and serve the purpose you would like.
Chestnuts can be "peeled" back if they are obtrusive. Use mineral oil to soften the tissue, and then gently remove it. It should peel off in layers. If the horse acts uncomfortable, stop, and take a little bit more off at a later date. You will not completely remove the chestnut, but if it is sticking out a long way, you can get is closer to the surface of the leg and make it less obvious. This method will reduce the amount the chestnut sticks out, but will not change the length of the chestnut on the leg. That is genetically predetermined.
