Metro, Metro Design, Windows 8, User Experience

The proper use of font sizes, weights, colors, tracking, and space can help give your Metro style app a clean, uncluttered look that makes it easier to use.
To create a consistent experience with other apps, check the guidelines for text and typography for Metro style apps. If you use your own fonts due to branding, still follow a font size hierarchy and the other guidelines.

Reference (HTML)

http://msdn.microsoft.com/en-us/library/windows/apps/hh700394.aspx

Reference (XAML)

http://msdn.microsoft.com/en-us/library/windows/apps/hh700394.aspx