RadChart does not support width in percent. It is rendered as image and it should have its size calculated at render time and unfortunately this is not possible if its dimensions are provided in percents as the actual size may vary, depending on various client-side conditions.
In addition to Shinu's suggestion, you can check the approach shown in this code library entry
. You will find projects for sever-side and for client-side resize. Basically, the main idea of server-side implementation is to find out the with of the chart container and set chart's width in pixels.
I hope this helps.
All the best,
the Telerik team