According to the U.S. Department of State, passport photos must be 2 inches wide by 2 inches tall. The photo must be proportioned so that the head is between 1 inch and 1 3/8 inches (between 25 and 35 mm) from the bottom of the chin to the top of the head. Using a professional portrait service is recommended but is not required.
In addition to the size requirements, the Department of State requires that the photo be in color, printed on either matte or photo-quality glossy paper and be taken in front of a white or off-white background. It must have been taken within the last six months and reflect the subject's current appearance. If a person has lost or gained significant weight or dramatically changed his hair color, he may need a more recent photo. The photo must also show the subject's entire face, both eyes must be open and the subject must bear a neutral facial expression. Clothing must reflect what the applicant might wear on a daily basis.