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It is not possible to find the pixel size without additional information.
Q: In the movie Star Wars, how large is the Imperial Battle Cruiser?
A: About 3 feet long. It just looks huge because of the camera angles and very very careful use of perspective.
If you have DICOM images it is often possible (but not always) to find the pixel size information in the DICOM headers. Some scientific devices put sufficient information into JPEG or TIFF headers, but this is not common and tends to be wrong.
If you have an object of known size in the image, such as a ruler, then it may be possible to calculate the pixel size.