They're going to be anal about it. Go onto here https://travel.state.gov/content/dam/passports/FIG_cropper.swf and edit your photo to get an exact fit!
@"Edward__" said:
I used a child's pencil - MY FIRST TICONDEROGA, they are much fatter than normal pencils - which reduces stress on your hand and also gives you a larger eraser.
I kept them in my front jacket pocket for easy grabbing.