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?
Pixel size is usually (beamsize/3). Based on the axes and number of pixels, you can find out pixel size by doing pixel_size = image size(in arcsec) / number_of_pixels.
Usually, the image is done by first finding the beamwidth of the telescope and then putting 3-5 pixels inside a beamwidth in any axis.
You can estimate the pixel size just from the number of pixels if you know the scale of your image or certain object in the image, but really you should already know this and the beamsize (which you could take a guess at but can't be sure). This information should be given with the image by whoever actually made it. If you have a data file it may be in the headers, or if you got the image from a website check the caption for details or an original source.