A method and apparatus can be used in the selection of correctly sized respiratory interfaces. The method includes the use of light beams to project sizing markers onto a patient's face, which enables the size of the patient's facial features to be measured relative to a set of interface sizes. A sizing apparatus can include a light source and beam splitters that will refract and project the light source into a series of beams that can be aligned on a patient's face in order to measure the features of the patient.