I have created a drawing, where the CENTERS of the most outer lines have a distance of 3.06mm in both X and Y.
The line weight of these outer lines is 0.6mm, so the whole image is 3.06mm + 2* 0.6mm/2 = 3.12mm wide and long (center to center plus half of the line weight on every side).
Additionally in the export there is a frame around the image, making it 3.12 + 2*frame.
The export function however calculates 3.06mm when setting the resolution to 1000Pix/cm, so it does not include the line weights and the outer frame.