Weird. Your font size is “7” and mine is “9”, but your text is slightly larger (I compared the Jimmie points). FR code must limit font size when you pick something larger than “7”.
<font size=9> 50mm</font> 50mm
<font size=9><big> 50mm</big></font> 50mm
You're right, but you can defeat it with the <big> command. You can put multiple bigs in it too:
<font size=9><big><big><big> 50MM</big></big></big></font> 50MM