Hi,
Try the following
Base page
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:Panel runat="server" Title="Panel" Header="false" Border="false">
<AutoLoad Url="Child.aspx" Mode="IFrame"></AutoLoad>
<Listeners>
<Update Handler="var b = this.getBody(); this.setSize(b.Ext.getBody().getSize());" Delay="500" />
</Listeners>
</ext:Panel>
</form>
</body>
</html>
Child page
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
</head>
<body style="padding:10px;">
<form id="form1" runat="server">
<ext:ResourceManager runat="server" />
<ext:Panel runat="server" Title="I am big panel" Width="800" Height="800" />
</form>
</body>
</html>