After my previous article about default styles and templates of XAML controls, here is the next set of useful MSDN resources for a Windows 8 app developer/designer.
Those two pages give the golden rules of a Windows Store App design:
Following those guidelines will ensure you have a sleek UI, that is consistent with the other apps.
The documents use the notion of unit, which is a 20 pixels space, and can be divided in 4 sub-units of 5 pixels each.
In a standard page, the top region containing the header will be 140 pixels (7 units) and the left margin will be 120 pixels (6 units). The bottom margin will be between 50 and 130 pixels (2.5 to 6.5 units) depending on your content.