I strongly advise you to bookmark this article, or the article it links to or just save the graphic from the link to your desktop. It's a handy tool for when you are creating new graphics for social media platforms.
The most often thing I look up when creating a Facebook page for clients or for my projects or other pages for other projects within Twitter or LinkedIn is "what are the dimensions I need for this image again?" And then either googling it or trying to find it on the site I'm working on so I can create an optimized image for it.
Well this site has all the dimensions in one easy to find infographic. Check it out here: http://www.mediabistro.com/alltwitter/social-media-image-dimensions_b44957
Bookmark it. You'll thank me later.
Image above also links to the article and is used in promotion of the article and is in no way affiliated with this blog except to promote the article.