An inch was a unit of length.
The Builders resembled eight-inch slugs. (PROSE: Primitives)
The Life Knife was eight inches deep by three inches wide. (TV: Everything Changes)
Princess Cilia used Ulian shrinking technology to make heself six inches tall and hid on top of a Christmas tree, pretending to be an angel. (PROSE: The Doctor on My Shoulder)
The Vinvocci, Rossiter, was one inch taller than the Master in his "Harold Saxon" incarnation. The Tenth Doctor noted this when Rossiter was disguised as a member of the Master Race. The Master failed to notice this difference in height. Rossiter knocked him out with his rifle. (TV: The End of Time)
The hull of the Octopus Rock was made of seven inches of steel. (AUDIO: Submission)
At twenty-seven years old, Janet Rae Tanner was five foot nine inches and a hundred nineteen pounds, as noted by Jack Harkness. (TV: The Middle Men)
One of the diseases created by the Viyrans was a metallic-looking worm which was six inches long. (AUDIO: Urgent Calls)
The typical psychic worm was less than an inch long. (AUDIO: The Wormery)
Uncle, the servant of House, having been damaged over time and repaired by the body parts of dead Time Lords, had a right arm two inches longer than the other when he died. (TV: The Doctor's Wife)
In 1991, three inches of steel lined every wall of the Cabinet Room inside 10 Downing Street. (TV: World War Three)