Having this kind of information before the operation can prevent problems later. A recent study showed tendon width makes a difference in the first months of rehab after ACL repair. However, two years later there's no difference in final results.
When the graft tendon is too small, the doctor can choose an alternative site for donor tissue. This can come from the hamstrings tendon and/or from the other leg. Graft material is often taken from the opposite leg when there is any indication that the injured leg is weak.
Quadriceps muscle weakness before surgery is linked with slower return of strength after the repair. This could be important if you're an athlete hoping to get back on the field.