I am aware of this behavior and it is a side effect of our custom RealFontSize
tool implementation. Basically, when you choose some value from the FontName dropdown it is inserted an empty font tag in the editor. After that when you select a value from the RealFontSize tool, the tool sees that the selection is empty and strips the Font tag, because it is not designed to work with empty selections.
If you do not like this behavior you can replace the RealFontSize tool
with the browser's FontSize
tool. You can see how to add new tools to the toolbar in this article: http://www.telerik.com/help/aspnet-ajax/addingstandardbuttons.html
All the best,
the Telerik team