What size this image should be depends on where it is to be used.
It is up to applications, eg WhatsApp, Facebook Messenger, Reddit, etc etc to decide what to do with the image. Some use it as a square image, some as a 1:19.1 rectangle, and some use different sizes when the display environment is of different sizes even for the same application. There are no rules, and there is no way to control what applications do with the image.
So you should test out the image on your intended target application(s) and find a compromise. That applications tend to cache the image makes it a pain on the trial and error approach.