Well, first of all, you are very, very strongly adviced not to do so. User's have theme and preference settings which they won't like losing. For example, 6 years ago I would make my screen resolution really high and use small fonts-- since then my eyesight has gotten worse and my font sizes have gone up.
GTK+ deals with the defaults of fonts, colors, and sizes through RC files.
Here it is in the tutorial: http://www.gtk.org/tutorial/c2118.html
And herer it is in the API: http://developer.gnome.org/doc/API/2.2/gtk/gtk-Resource-Files.html
However, if you must change the font and size, you do so with Pango
PangoFontDescription *fd = NULL;http://developer.gnome.org/doc/API/2.4/pango/pango-Fonts.html
GtkWidget *w = NULL;
/* code here to create widget or get it from glade */
/* change the widget's font */
fd = pango_font_description_from_string ("Monospace 12");
gtk_widget_modify_font (w, fd);
/* display the widget or do whatever else you gotta do */