Thank you for writing. You have run across a known issue with the TextPrimitive that has been used in RadMessageBox
object. We are going to address this in the next release (it is possible to include a fix in the next internal builds as well). Currently, you can place the <html>
tag before your string. If this does not help, you can try to manually format the text in a couple of rows by including new lines in your string.
the Telerik team