a good place to start would be Microsoft's own Windows Phone User Interface Design and Interaction Guide. This is instructive because it tells us what Microsoft is thinking about finger-based interfaces. One key detail tucked away in there is that any touch target should be at least 9mm×9mm (typically about 34 pixels square) to ensure that it's big enough for a finger to hit. In exceptional cases, they can be a little smaller; 7mm square, about 26 pixels square. This guideline is a simple one, but it's important. With UI elements smaller than this, the user interface simply isn't convenient to use with fingers.
Windows 7, however, is chock full of elements smaller than 9mm. Taskbar buttons and Start menu entries are big enough, but they're exceptional. Icons in the notification area, title bars, toolbars, scrollbars, checkboxes, radio buttons, everywhere you look, there are pieces of user interface that are too small.