If you're building a Windows Phone Store app (meaning an appx app, not a xap app) then you may find this info about visual assets useful. It describes the sizes, padding, scaling, backgrounds, etc, for various visual assets. Inside the attached file ("Windows Phone Store apps visual assets.zip"), you'll find individual sample assets, which have transparent areas of padding where applicable so that you can use them as templates to build your own assets.

The scope of this set of assets covers: Start screen tiles, application and Store icons, Splash screen, app bar buttons, Hub background, and file type and protocol association icons. Click on each image below to enlarge it for reading.