I finally did like below:
- Convexe Hull
- Centroid of this Convexe Hull
- minimum distance between Convexe Hull points (thanks to
exterior_ring
function) and the centroid
By the way I don't know why I had to take the minimum. It was to adapt my scale. I don't really understand how the centroid of a polygon (which could have really strange shape) can be found.
Anyway, it's working thanks to the 3 steps written previously.