Real Area of a picture

I want to know the actual area of the entire area covered by an aerial photograph. _The Photo is 4000x3000 pixels _The Focal Length of the camera is 20mm. _The Picture height is 51 meters.

I researched and got the scale of the photo. In this case it is 1 mm / 2550 mm.

I understand the scale tells me that if I move 1 mm inside of the photo , in the real terrain I would be moving 2550 mm. I want to know how to get the real width and height of the picture. ¿? cm. I need this data to know the area of the photo and therefore know the terrain area that is pictured.

Thank you for your help.

If you ignore distortion and curvature, the physics is qute simple. There are two similar triangles in the picture below:

You know the focal length (20mm), and the distance from the focal plane to the scene (51m). So you need the width of your sensor's CCD to put into the equation:

sensor width       focal length
------------    =  ------------
scene width        camera height

Make sure all the units are the same, i.e. don't mix millimetres and metres.