Is it possible to get font size in any apps in OS X?
WhatFont is not as magical as it seems - it's simply surfacing information already present in the web page coding (the font being used) in a way that is easy for non-technical users. There is no program that I am aware of that will identify fonts under the cursor when the information is not already available.

MyFonts has a service called WhatTheFont! where you can submit a screenshot and it will try to identify by matching your image against known samples in their database.