This is a known issue for us and it has already been fixed in 2012 Q3 version of the control, so you could give it a try. If you are not able to upgrade your project, you could use as a workaround BeginUpdate() and EndUpdate() methods of DocumentSelection after changing of the font size or when ChangeFontSizeCommand is executed:
.radRichTextBox.CommandExecuted += (sender, e) =>
I hope this solves your problem. Do not hesitate to contact us if you have other questions.
the Telerik team