How to change font size

["windflying zhou" <windflying zhou gmail com>]

Hello, I'm developing an application with gtk+, and I'm trying to
 change the fonts that is used in the application, to use diferent
sizes of letters. I didn't find any class or method that changes the
properties of the font used by the application. So I don't know if it
 is possible to change the fonts of an application, and if it is
possible wich class can do it.

Thanks for the help.