Hi Chandra Prakash,
You can use the FontSize property of the rich text box to alter the default font size used in the control. If you are using this approach you will also have to set the DocumentInheritsDefaultStyleSettings to true. Here is an example:
You can also change the font size runtime, using the IncrementFontSize and DecrementFontSize methods in the RadRichTextBox. Here is a sample button event handler that will decrease the font size of the entire document:
sender, RoutedEventArgs e)
I hope this was helpful. Please contact us if you have more questions.
the Telerik team