This program show a simplified view of what size an object has with 3 different focal lengths.

Focal length n°1 (mm) : 400  // write the focal lengths you have
Focal length n°2 (mm) : 750
Focal length n°3 (mm) : 1500
Size (height) in arcsec : 40 // write here the diameter of your object (here Jupiter)
Size (length) in arcsec : 40

This program is designed for a sensor with 3.75µm of pixel size and with 6000*4000px resolution. You can change that in the code respectively lines 17 and 12 - 13