Final answer:
To estimate the proper size of an OPA, hold it against the patient's cheek and measure from the mouth's corner to the jawline angle, ensuring a fit tailored to the patient's anatomy.
Step-by-step explanation:
The best way to estimate the appropriate size of an oral pharyngeal airway (OPA) before insertion is to hold the OPA against the patient's cheek. This method involves measuring the airway device from the corner of the patient's mouth to the angle of their jawline. This estimation technique takes into account the individual anatomy of the patient, providing a custom fit for better airway management.