A servo arm that is longer than the control horn (measured from the pivot point to the hinge line) will create a mechanical DISadvantage at the servo. This is typically not the best set up as you are lowering the available torque of the servo. Keeping them the same length is the optimum set up for pull-pull. In regards to the offset, you would only need offset in the servo arm if there is any offset at the control horn present (pivot points substantially behind the hinge line). The control horn pivot point should be exactly on the hinge line or very slightly behind it, never in front of it. Set up in this manner, a straight servo arm of the same length as the control horn would be the optimum to achieve the correct geometry.