As short as possible that will still give you the required control surface throw.

I doubt any of my jets have servo arm lengths more than 3/4". If you need really long servo arms to get enough control surface throw it is quite possible that you could use shorter control horns at the surface, or that your control horns are mounted improperly (i.e. horn mounted too far behind the hing line).

The control horn ideally should be mounted so that the clevis hole(s) are on the same plane (inline) with the hinge line. It is best to have it supported between two of the hinges, and it should not be any longer than necessary to insure no movement if the pushrod was absolutely locked down. Then the servo arm should be as short as possible to achieve the required throw.

The longer your servo arm is any slop in the servo will be amplified at the control surface.
Using the shortest arm possible also allows you to us the full travel of the servo which will give you better resolution at the control surface than you would get if you used a longer arm and then had to adjust the control surface throw by cutting down the servo travel in the transmitter.