Thank you for writing.
You can change the font by assigning a Font variable to the Font property of RadTextBox. Consider the following code snippet:
Form1 : Form
, 14, FontStyle.Bold | FontStyle.Italic);
Note that it is a good idea to have fonts created only once in your app, because the creation of a font is a heavy operation and can lead to memory leaks.
I hope this will help. If you have further questions, I would be happy to help.
the Telerik team