Is there an effective means to measure the dimensions (length, width, diagonal length) of the effective area of an image sensor in a camera from a photo?
Like many problems with multiple variables, as long as all but one element of an equation is known, then it may be solved for the final variable. If the focal length, focus distance, and size of an object at that distance are known, it would be possible to compute the size of the sensor of the camera that took the photo. That is assuming no post exposure cropping was done.
Likewise, if you know the pixel pitch and resolution of the sensor you could compute the overall dimensions of the sensor.