I recently needed to size an image, using Adobe Photoshop CS, to a 1:1 scale.  This was needed because I wanted the image to print out in its 'actual size'.  Photoshop probably has this capability built in somewhere as a tool, but I haven't seen it.  For sure, there are some third party tools out there, but I stumbled through a no-frills, simple approach using just Photoshop's built-in toolkit.

Since I had a scale (a ruler) in the actual photograph, I was able to use this method to modify the scale of the image to make it 1:1.

• Open the image file
• Click Image>Image Size
• Change the 'Width' dimensions to 'mm' if it isn't already set that way
• Make a note of the width (in my example, the width was 1,587.5mm)
• Close the Image Size Dialog
• Switch to the 'Measure Tool' (it shares a button with the 'eyedropper tool'
• Using the Measure Tool, measure the scale (the ruler) in your image as exactly as possible.  In my example, I was able to measure 23mm on the scale.
• Make a note of that measurement, and of the distance Photoshop says you've measured (in the top bar, there is a 'D1' measurement; in my example, it was 1,035.2)

Now it's down to simple math.  You can figure out the ratio by which the whole image's size needs to be scaled by dividing the distance Photoshop says you measured by the measurement you took.  In my example, 1,035.2/23 = 45.001.  So the reduction factor I need is 45.001.

• Take the width of the image (mine was 1,587.5) and divide that by your reduction factor (mine was 45.001).  That result will be the proper width of your image (mine was 35.271)
• Click Image>Image Size again.
• Make sure the 'resample image' checkbox at the bottom of this dialog is 'unchecked' (otherwise, the quality of the image will get hosed)
• Enter your new, proper, value for the width of the document
• The document's height should change too...keeping the image's scale intact
• Click OK, and your image is now sized 1:1!

As I said, this worked for me because I had an actual scale in the digital image to use for measuring.  As long as the photo is taken at a right angle to the face of the subject, you should be able to use anything in the photo that is of a known distance.  For example, you could follow this method using the diameter of a penny if one was visible.