This study from Korea examined the accuracy of oropharyngeal airway sizing based on measurement from either the maxiallary incisors or the corner of the mouth to the angle of the mandible.
I've always thought it was measured from the incisors to the angle of the mandible, but perhaps there is another school of thought on the matter.
They found that measuring from the incisors provided a better airway for ventilation with a BMV, with 6% of the corner of the mouth group having partially obstructed ventilation, and 7% having inadequate ventilation. In addition, examination with a bronchoscope identified a significant portion of the corner of the mouth group to have complete or partial tongue obstruction (too short), although 22% of the incisor group extended beyond the epiglottis (too long).
Measure up for your OPAs from the incisors to the angle of the mandible (as you likely already do) for a better airway for ventilation without tongue obstruction. Measuring from the corner of the mouth selects an airway that is too short.