Generally speaking, the screen resolution should not make any difference (except for the image comparison feature of Ranorex) as long as the UI elements of your application are accessible by Ranorex. This should be the case with Silverlight applications.
If the UI elements are not directly accessible, e.g. if a control is owner-drawn and do not provide any information by means of MSAA or other standard interfaces used by Ranorex, relative screen coordinates are used as fallback. In this rare case the screen resolution could matter indeed and you should try to improve the accessibility of your application.