I just put together this landing page template, the design is very popular especially if your promoting a single product, the background is set to no-repeat which gives it that steady effect. You can pretty much use any size background but my recommendation would be to use something under 1000px for a quick load time.
I included iPhone PSD with a smart object layer. Please see at help document on how to change the background image. See the demo below, and don’t forget to like TechAndAll on facebook