Hi Klemen Slavič,
The width of the resize / dragzones of RadWindow is what is being set by the person that creates the custom skin. These zones are also part of the elements that make up the design of the skin and cannot be controlled separately. What I mean is that you cannot set the left rezize bar to 10 pixels to fit the image that you are using for this part, but to set the resize zone to 4.
the Telerik team