A Mexican passport photo is 4.5x3.5 cm. However, you don't want a regular passport photo.
For the PGR informe which you are trying to get (since Mr. mc1234 is in Canada), the essential requirement is that nothing is covering your face and that the photo is taken very, very close up to use up the entire frame. They prefer it to be at least 4.5x5 cm so Canadian passport size works fine (5cm x 7cm) - in fact, when the Canadian Embassy processes it they request Canadian passport photo size.
You can have no: makeup, earrings, glasses, beard, mustache, sideburns and all hair/bangs must be pulled back. The forehead and ears must be fully visible and uncovered.
Also note that they request 3 photos of the front (remember: as zoomed in as possible) and 3 photos of the right profile (remember: as zoomed in as possible).
For others who may read this, for constancias (if you go in yourself because you are in Mexico) you need to get fotos tamaño filiación (they are oval, just like diploma photos and have very specific dimensions required for the face and profile which photographers in Mexico are familiar with).
ETA: You can call them at 5346 0000 Ext. 8428, 8430 or 8427 to confirm since I'd want to check it directly too