Hi Vasssek,
The following KB explains how you can set a RadWindow's size in percent: http://www.telerik.com/support/kb/aspnet-ajax/window/details/setting-size-in-percent-for-the-radwindow. The radalert() method returns a reference to the RadWindow it creates so you can store and use it.
You may also find interesting RadNotification for showing messages to your users, because it supports percent units out of the box: http://demos.telerik.com/aspnet-ajax/notification/examples/sizeinpercent/defaultcs.aspx.
Regards,
Telerik by Progress
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