How to calculate the pixel size and beam size of an image? I have an image with some sources in it. I need to get the pixel size and beam size of that image. How can I calculate it?
If you know the angular size of your image (e.g., 10", 1' etc), then pixel size  (in "/pixel units) is just image size / number of pixels.
For beam size, depends a lot on the wavelength-range of your observations (for instance sub-mm/radio observations are conceptually very different from UV/optical), but to have an idea of the beam/psf size, you can fit a 2D gaussian/moffat profile to the emission of a point-like object (i.e., a star) in the field of your image.

Note that most of the time these info are contained in the header.