Best barrel length depends a lot on the type of action you are using. If a single shot, and if you want to shoot it fast, the longest barrel that carries with good balance would be my choice. This because at high velocity, barrel time is short enough that slight movement between the time the trigger breaks and the bullet exits won't effect accuracy too much, and long barrels move the blast farther from the ears, making shooting a lot more pleasant.
If a short barrel is desirable to you for brush or horseback carry, you won't go wrong with a barrel as short as 16 inches, so far as velocity and accuracy. In any case I hate a gun that's so short that it wobbles easy, unless it is intended for very close range snap shooting in brush and such. Your smith will probably have to buy a 22 inch barrel, and if he does, and you can get together with him, try it for balance, trimming the length till it handling qualities make you happiest. If you love the feel of the gun, you'll be able to hit anything you point it at. If it feel like a club you won't carry it.
