background image dimensions

Hi, quick questions. I’m trying in vain to set up a bespoke background image on the Intuition theme. Its loading no problem, but can someone let me know the dimensions the image should be. ie, I have a photoshopped image of the sea behind the theme, but when i upload it, only parts are showing, despite numerous attempts at placing the items in different locations on the image before uploading.

cheers in advance!