This has information about the size and dimensions for the logo on SocialCommand
The logo dimension recommendation would be 450px x 75px , 900 x 150 or 1800 x300. Something in that same general shape. As an example, a "good" logo might look something like this one:
The important things to note would be the shape and also to make sure that your business name is included in the logo since once you upload one the text that is there right now will be replaced by your logo.