Hi Derek,
Thank you for writing.
You can achieve your goal by inheriting from RadPrintPreviewDialog and overriding the OnShowPrintDialog method. This method is called when the user clicks the print button. There you can change the Landscape property of the printed document. Here is the code you can use:
To show your dialog you should use the following snippet:
I hope this will be useful. Should you have further questions, I would be glad to help.
Regards,
Ivan Petrov
the Telerik team