The distance from a pivot point (the CG) to where a force acts (stab, fin etc.). A longer tail section means a longer moment arm. The "moment" itself is the force times this distance. The same stab on a longer or shorter moment arm gives a greater or lesser moment, or stabilizing force. In addition to this, imagine a longer tail boom versus a short one - the distance the stab or fin has to move for each degree of pitch or yaw is greater = more damping due to inertia = more smoothness, less twitchiness.

Dunno if that helps or confuses, I tried.