Radwindow auto size is not working when content of radwindow (seperate page) present in updatepanel

I used radpanelbar inside radwindow when changing one panelitem to another panelitem the size of the radwidow is not changing .Actually the content of second panelitem is more than first panelitem . I placed radpanelbar inside updatepanel.
when writing  postback triggers for radpanebar it is working fine as it makes full postback.

I dont want any postbacks in my radwindow.