Take a look at the enumerators at the end of this page: http://www.telerik.com/help/aspnet-ajax/window-programming-server-side-properties.html
. There you can see the possible values the Animation property can take. I believe Resize is closest to your requirement (left top corner to bottom right corner), while now you are using Slide (which is top to bottom). Just test them out (you can also use the intellisense in the markup) and see which fits your needs best.
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