A square millimeter (mm2) was a unit of area equal to one millimeter by one millimeter square.
In preparing for the first head on encounter with the Borg in the end of 2373, The Doctor analyzed every square millimeter of the Borg corpse USS Voyager had recovered. (VOY: "Scorpion")
After Tom Paris finished covering for The Doctor in sickbay in 2374, The Doctor wished to have Paris help sterilize every square millimeter of the sickbay. (VOY: "Revulsion")