How large is considered a abnormal jaw line?

Jaw Alignment: This can't be easily answered. What is considered large in one person is small in another. This is why an individual evaluation is important. Measurements are made with an x-ray and then those are compared to norms. The different measurements made are too many to list. Not to mention that there are different norms for different ethnicities.

Jaw line: It depends on the rest of the face.

Depends: The jaw line, usually referencing the projection of the lower jaw forward is actually the lower most part of the mandible. The "length" of it is relative to the remainder of the face, and the angle which this line makes with visual axis (line from eye to ear). A high angle makes the jaw line seem smaller and vice-versa. Ethnic norms and the subjectivity of the viewer make it normal or not.