create a new blank image 2 in x 2 in at desired resolution (300 ppi)

place horizontal guide lines for passport criteria (1/2 in and 1-1/2 in) for reference. Place a single centered vertical guide.
copy/paste your original portrait (assuming it is not 2 x 2) image as a new layer

Transform/scale the portrait layer until the face fills the area between the horizontal guides and centered over the vertical guide.
Clear guides and flatten for printing.
forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=27505832