The RadFileExplorer control is based on a several complex controls and managing the size of the controls is very hard to be implemented. That is why, the control can be used with fixed values only (set in the RadFileExplorers declaration) and its size cannot be changed dynamically and this restriction is by design and cannot be overridden. Note that the RadFileExplorer control does not have set_width() or set_height() methods.
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the Telerik team