A picture is worth a thousand words, and nowhere is that more true than on social networks. This is why it is incredibly important that the media you use on social networks fit the standard sizes.
The following infographic tries to list all the standard image and video sizes for the major social networks as well as Google Plus. It's really useful, but you should still take it with a grain of salt; I know that at least one of the images sizes for LinkedIn is no longer correct.
So caveat emptor.