Yes, you can easily change the width of a RadPane on the client by using its client API and the set_width method in particular which is listed below:
All you should do is to obtain a reference to the particular RadPane either by using the $find method or through the splitter client object and call the set_width method. Note, however, that you can change the width of a RadPane only if it is in a splitter which is with vertical orientation because when it is horizontal, you should change the width of the whole splitter.
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the Telerik team