Design item Metro App for Windows 8: Leverages typography and whitespace, and follows a font hierarchy
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)
https://msdn.microsoft.com/en-us/library/windows/apps/hh700394.aspx
Reference (XAML)
https://msdn.microsoft.com/en-us/library/windows/apps/hh700394.aspx