Do you know what a pixel is?

--- [from wn] ---
pixel
     n : (computer science) the smallest discrete component of an
         image or picture on a CRT screen (usually a colored dot);
         "the greater the number of pixels per inch the greater
         the resolution" [syn: {pel}, {picture element}]
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So to translate pixels to millimeters you first have to know your resolution. I hear most common resolution is 72 pixels per inch.