There are many photo imaging software programmes from expensive - Photoshop CS5 to free. If you have a programme I would suggest a resolution of 300 dpi, and then resize to what you need - try a few sizes a mm or so apart. No doubt someone will suggest a freebie, such as the Gimp.

Important always work on a copy not the original

You could use ifranview its free import your image Go to edit> create custom crop selection> onthe right hand side of the box that opens just chose inches and enter 1 in the width and height window enter 1 amd 1 or whatever size is needed to enclose the face .Now click on apply to image.You will have a square now on the main image move that (holding the right click ) over the area required. Now holding the leftclick over any ot box edges till you get a doubleheaded arrow drag each line in till it covers just the area required You can now choose the actual size by going to edit ]1Resize /resample tick set new size enter your sizes . Best to avoid changing any aspect ratio if possible As said previously use 300 ppi as the resolution .

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Paint.Net is also a good one. Always work off a copy as said above.

resize photo

Save the file to PDNet (rename) as default PNG. Make your changes and save as 8-bit BMP file. BMP files are the best files not to lose colour hue and sharpness.

I agree with polymath's last post but hope the experts can expand on the info.

Sorry Yimbo my error once you have the selected area around the face and you have sqeezed teh lines in as much as needed go back to edit >crop selection then go to Image (not edit) and resize /resample will be there