VB/Solution/Default.aspx

<%@ Page Language="VB" AutoEventWireup="true" CodeBehind="Default.aspx.vb" Inherits="Solution.Default" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript">
        function CalculateSize() {
            var iframeElement = splitter.GetPaneByName('ContentUrlPane').GetContentIFrame();
            var iframe = GetFrameOfIFrame(iframeElement);
            var doc = iframe.document;

            splitter.SetHeight(doc.documentElement.offsetHeight);
        }

        function GetFrameOfIFrame(iframeElement) {
            var name = (new Date()).toString();
            var backup = iframeElement.contentWindow.name;
            iframeElement.contentWindow.name = name;
            var frameIndex = this.internalGetFrameByWindowName(name);
            iframeElement.contentWindow.name = backup;
            return window.frames[frameIndex];
        }

        function internalGetFrameByWindowName(name) {
            var count = window.top.frames.length;
            for (var i = 0; i < count; i++) {
                if (window.top.frames[i].window.name === name)
                    return i;
            }
            return -1;
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <dx:ASPxSplitter ID="ASPxSplitter1" runat="server" ClientInstanceName="splitter"
            Width="100%">
            <Panes>
                <dx:SplitterPane ContentUrl="ContentPage.aspx" Name="ContentUrlPane">
                </dx:SplitterPane>
                <dx:SplitterPane>
                </dx:SplitterPane>
            </Panes> 
            <ClientSideEvents PaneContentUrlLoaded="CalculateSize"/>          
        </dx:ASPxSplitter>
     </div>
    </form>
</body>
</html>