Measuring shapes with Raspberi Pico cam(Drone competition(

It is for a students drone competition, we are allowed to get outside help. The challenge is measuring precisely a star-like shape using a drone camera, it must be more then 1,2 meters away from the ground. We are thinking of using rasp pico but we are struggling to find a way to get the scale of the picture, so we can calculate the area.

1.5 meters*