What are the screen dimensions given to my app?

All Windows phones currently have Wide Video Graphics Array (WVGA) screens that are 480 pixels wide and 800 pixels tall. Your app can consume all of this space if you are not showing the status bar or application bar. In the portrait orientation, the status bar consumes 32 pixels of height and a fully-opaque application bar consumes 72 pixels of height. In the landscape orientations, the status bar and application bar each consume 72 pixels of width because they don’t move from their original locations, as seen in the preceding chapter.

Another consideration when designing your user interfaces is that toast notifications, when they appear, temporarily cover the top 60 pixels of the screen. If your app is used in the midst of a phone call, a bar with call information covers the top 64 pixels of the screen. When the user adjusts the phone’s volume or interacts with the audio transport controls, the resulting volume control covers the top 93 pixels. This is one of the reasons that the standard page design of showing the application name at the top works out nicely, as only that gets covered in these cases.

That said, it’s best to avoid depending on the specific screen dimensions. Future phones will undoubtedly have different dimensions. You can dynamically discover the resolution with the following two properties:

(Application.Current.RootVisual as FrameworkElement).ActualWidth
(Application.Current.RootVisual as FrameworkElement).ActualHeight

or:

Application.Current.Host.Content.ActualWidth
Application.Current.Host.Content.ActualHeight

You can also discover the current page’s dimensions (minus the status bar and application bar, if applicable) by checking the page instance’s ActualWidth and ActualHeight properties.

If you wish to perform a custom action when the orientation changes, such as a full-screen animation or custom rearranging of elements, you can leverage a page’s OrientationChanged event.

Source: http://windowsphone7reviews.net/101-windows-phone-7-apps-in-case-of-emergency