Using your favorite image-editing software, open the file, change the canvas size of the image and then print it.
Remember, there are a lot of guidelines other than just size that a passport photo must meet as demanded by the government. Size of the head, for example, is of key importance. The length from the bottom of the chin to the top of the head must be of a specific size. No shadows behind the person's head and even lighting are things, that if not met, will get your passport declined.
Considering the time and effort involved in getting a shot to meet government guidelines, it's far easier and cheaper to spend the $20 and just have someone take your photo.