When a plane is not level and not turning, a sideslip condition occurs. A sideslip is another name for yaw. The yaw increases the angle of attack of one wing and reduces it for the other wing. This causes a rolling force that levels the plane. If the vertical stab has too much area, it will weatervane the fusilage to line up with the airstream and that self righting tendency is reduced. Such a plane is spirally unstable. If not corrected by the pilot, the plane will go into a steeper and steeper bank as it dives more and more going into a spiral dive.
If the vertical stab doesn't have enough area, the side slip yaw will roll the plane back to level but the plane will overshoot level flight and then re-overshoot level flight the other way, and on and on. This is called "dutch roll".