» What is the best size for the background picture?
C5 Extras > Support > Knowledge Base > Express Theme > What is the best size for the background picture?

You can use any size that you want for the background image. The theme will automatically stretch it for you. However, if you want to make it look like it's not stretched or lose the quality of the photo what you can do is make sure that your image has 2600px by 900px dimension width and height respectively.