As per your requirement, i would say that as mentioned above your query, for size of certificate upload in jipmer pg, while filling up the form and uploading the image, itself it will be given there the size reauired for uploading the image. According to that, fill it up.
You haven't mentioned your Category. Neverthless, JIPMER has Caste Certificates in a certain format available. You need to download it, fill it up and upload the document whenever applicable. You can access the caste certificate format on Pages 38 and 39 from : http://jipmer.edu.in/sites/default/files/MD%20MS%20Prospectus%20January%20%202020%20session%20Final_0.pdf