Comma separated list of class names that is to be used when the user selects font sizes, this option is only used when the convert_fonts_to_spans option is enabled. This list of classes should be 7 items. This option is not used by default, but can be useful if you want to have custom classes for each font size for browser compatiblity.
Example of usage of the font_size_classes option:
tinyMCE.init({
...
font_size_classes : "fontSize1,fontSize2,fontSize3,fontSize4,fontSize5,fontSize6,fontSize7"
});