Biro with reversed 18-55mm kit lens set to F11 at 18mm and close focus
and 62mm of extension tube.
The reversed lens and extension tube gives us a 8:1 macro (X8)
The diameter of the ball point is 1 mm

You set the lens for F11 whilst on the camera in Av and then pressing "stop down preview".
You then remove the lens whilst holding the stop down preview with the camera still on.
See previous three images for comparison of size.