I remember it used to be something like 320px by 300px (with also specific dimension in cm or inch).
This time, it says only the resolution to be 600px by 600px. Its ok to make the eye height and head height as mentioned in Link-1 for the resolution of 600px by 600px, but how about the dimension of the photo:
Height and Width in cm or inch ???  

Any idea?

Link-1:
http://www.dvlottery.state.gov/photo.aspx

LInk-2
http://www.travel.state.gov/pdf/T1026V-DV-2010bulletin(3).pdf