I just tried the following code which seemed to work fine for me:
sender, EventArgs e)
RadSlider sl =
sl.RenderMode = RenderMode.Lightweight;
sl.Width = Unit.Percentage(100);
Could you see if it is OK on your end as well? What is important is that the RenderMode is set to Lightweight before the Width property is evaluated.
Also, can you confirm you are using the latest version of the controls (Q2 2015 SP2 at the time of writing)?
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