This <b><u>should</u></b> be exactly 10 mm long, as long as the width of the image is not changed (zoomed), always assuming you're using a common screen or viewer. This will almost certainly show the wrong size on smartphones.
<a href="//commons.wikimedia.org/wiki/User:Perhelion" title="User:Perhelion">Oliver Albrecht</a>
<span class="int-own-work" lang="en">Own work</span>