I've just spent a couple of hours fixing up the cover images on my Facebook, Twitter and Google+ business Pages. Of course, they wouldn't make it simple and have them all the same size...
For anybody about to embark on the same exercise, here are the optimum image sizes for each platform:
Facebook: 851px x 315px
Twitter: 1252px x 626px
Google+: 480px x 270px**
(Click on the social media name above to see my end results).
If you need any assistance with your social media requirements, please don't hesitate to contact me.
**Updated 8th March 2013