On Mac there is the concept of Font Variations, where the system font can appear in different sizes based on intended usage.

For example a label should have a smaller text size than the label of a button. This can somewhat be emulated by explicitly specifying different font sizes. On Windows the variation of font sizes are not used, so for Windows it will be required to explicitly specify the same font sizes for buttons and labels.

It would be very nice if there was a platform independent way to specify font sizes, so that Font Variations would look properly on both Windows and Mac with the same code.

For further information please see:
A list of the constants Apple supplies for Carbon applications:
http://developer.apple.com/documentation/Carbon/Reference/Appearance_Manager/Reference/reference.html#//apple_ref/doc/c_ref/ThemeFontID

The way to change these constants into usable data
(font name, size, etc):

http://developer.apple.com/documentation/Carbon/Reference/Appearance_Manager/Reference/reference.html#//apple_ref/c/func/GetThemeFont