auto resize pannel according to content

Question

I had an pannel which contain the extender modalPopUp.dragcontrol extender and i written css for pannel and model popup

I am using this modelpopUp for multiple forms which contain diffrent contents 

i set pannel width and height as 800px and 600 px;

i need auto resize pannel so i can use same css for multiple forms. 

--pannel css-------------

.PopUp

        { 

    position:relative;

    background-color:#f3f3f8;

   border-radius: 7px;

   

   border-right-width:2px;

   border-left-width:2px;

   border-bottom-width:2px;

  

  

   border-bottom-style:groove;

   border-left-style:groove;

   border-right-style:groove;

   

   border-color:#f3f3f8;

    width: 800px;

    height: 600px;

    overflow:auto;

        }

---modal popUp-------------------

 <asp:Button ID="btnShow" runat="server" Text="Show" />

                  <asp:ModalPopupExtender ID="btnShow_ModalPopupExtender" runat="server" 

                  DynamicServicePath="" Enabled="True" TargetControlID="btnShow"

                  PopupControlID="pnlPopUp" BackgroundCssClass="backgroundModelPopUp" DropShadow="false"

                  OkControlID="btnSubmit" CancelControlID="btnclose" >

                  </asp:ModalPopupExtender>

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