To reproduce: 
- run the attached sample project
- click the 'show' button
- click the 'update text' button while the alert is shown. You will notice that the text is updated but the alert is now resized according to the new content.

Workaround: hide the alert and show it again in order to force recalculating the size. 

        Me.RadDesktopAlert1.PopupAnimation = False
        Me.RadDesktopAlert1.Hide()
        Me.RadDesktopAlert1.ContentText = Me.RadDesktopAlert1.ContentText & "Donec ornare, elit a lobortis luctus, mauris est interdum leo, eget malesuada ligula diam quis nunc."
        Me.RadDesktopAlert1.Show()
        Me.RadDesktopAlert1.PopupAnimation = True