You cannot got an accurate measurement of the length of an object from a single image easily.
The GSD is calculated by taking the camera's field of view, AGL altitude and sensor dimensions into account but that number is really only valid directly under the aircraft. The GSD changes as you get further away from the aircraft.
As long as the object being measured is relatively small within the field of view of the camera and near the center right under the aircraft counting the pixel should work.
Be careful where you are getting your AGL data since that will affect it greatly.
This measurement could be made in Maps Made Easy after processing the images into a map but that would require the crocs being still which might not work.