See Image>Print size. The "print definition" is just an arbitrary ratio of pixels/unit of length. It is sometimes acknowledged, once in a blue moon, to be the size at which you want the image printed, but it has other uses.
- With Gimp's measure tool (
) measure (in pixels) some distance/length in your image between two points for which you know the real distance
- A simple division (length in pixels / physical length) gives how many pixels to the cm/metre/yard/furlong you have in your image.
- Enter these values in the print definition dialog,
- Change the display unit in the small indicator at the bottom left of the image display.
- From now on all measures (Measure tool, but also rulers, guides...) are in physical units.