Hi Susanto Siman,
This behavior is by design - the way we have implemented RadRotator for ASP.NET AJAX - to keep the rotator parts - buttons/viewport, always in pixels, led to the decision that the rotator should not support setting these properties in percents. At least for the time being.
Generally, you can implement this yourself, by calculating the correct size of the rotator every time its parent RadPane resizes, but the set_width and set_height client methods of the RadRotator are not yet implemented and you can only set the Width/Height of the control on the server. That is why I cannot offer you a workaround for the time being.
the Telerik team
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