Another great guide to add to the list is their Always Up-to-Date Guide to Social Media Video Specs. They offer guidance in the following networks:
Head on over and give it a look here. They also have a Google Doc you can access that has the dimensions in a tabbed spreadsheet form. They even tell you how to turn on notifications so you can be alerted when sizes change. Make sure to pass these great resources along to whichever staffers manage your accounts so they can use it this year to help make your social presence as strong and professional looking as possible.