Rotator Reponsive design

The rotator itself may get 100% width, but this has some requirements and limitations (such as that the  Rotator's items have width in pixels) as described in the documentation: http://docs.telerik.com/devtools/aspnet-ajax/controls/rotator/mobile-support/fluid-capabilities.

I suggest you take a look at the ImageGallery control, which offers responsive and adaptive rendering, to see if it will fit your needs: http://demos.telerik.com/aspnet-ajax/image-gallery/examples/overview/defaultcs.aspx. It also offers templates if you need more complex content: http://demos.telerik.com/aspnet-ajax/image-gallery/examples/functionality/templates/defaultcs.aspx.

Here is a basic example:

<telerik:RadImageGallery runat="server" ID="RadImageGallery1"  ImagesFolderPath="~/images" Width="100%"></telerik:RadImageGallery>