Hi Moe....the recommended size would be 1920 wide and your height should be around 400 px or more (or less), depending on your preference. The one for the theme though is 1920 x 545.
If it's worth a 5 star rating and review, I can make your day by saying if you contact me from my styledthemes.com contact page, I can send you the Photoshop file with layers intact if you wish? ....ok, no obligation on the stars and rating, but I have no problem sending you the PSD for the background :)
