You can set predefined FontSize - Font Size dropdown - Sets the font size or appropiate tool of the RadEditor Telerik's control. More information at http://www.telerik.com/help/aspnet-ajax/toolbarintro.html.
However, if you are using Sitefintiy 3.6 builds you need to map the external templates. Another option is working with ToolsFile.xml located under Sitefinity/Admin/ControlTemplates folder.
If your approach is different, let us know.
All the best,
the Telerik team