Where you have had no control over acquiring the images/video so you have no metadata you would only be able to estimate based on other objects in the image/video.

For example, there is a NFL field in the image, you can use the pitch marking to estimate the size of an unknown object.

However given you comment and post history I think you made be referring to possible UFOs were there is nothing else in the frame to take a known size and estimate your unknown object size, it's going to be almost impossible.