Navigation:  Painting Tools > Measurement

Measuring tool can be activated through the main menu -> Paint -> Measurement -> ...

Or you can click at the following button on the Painting Tools panel:

(you can open this panel through the main menu -> Paint -> Show Paint Tools).

You can select the type of the measurement:   length, area, circle or angle.

After that you can measure on the image using the cursor

You can also select the measurement unit in the main menu: Paint -> Measurement -> Units -> ...