Hi,

AutoHeight is not always possible because some layouts require size for the container (like HBox, VBox, Fit)
Please see the following sample

Index
<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<%@ Import Namespace="Ext.Net.MVC" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title></title>
</head>
<body>
    <ext:ResourceManager ID="ResourceManager1" runat="server" />
    
    <ext:Panel ID="Panel1" runat="server">
        <Items>
            <ext:TabPanel ID="TabPanel1" runat="server" AutoHeight="true">
                <Items>
                    <ext:Panel
                        ID="homeTab"
                        runat="server"
                        Title="Home"
                        AutoHeight="true"
                        >
                        <AutoLoad Url="/Test/HomeTab" NoCache="true" ShowMask="true">
                            <Params>
                                <ext:Parameter
                                    Name="containerId"
                                    Value="function () { 
                                        return #{homeTab}.id; 
                                    }"
                                    Mode="Raw" />
                            </Params>
                        </AutoLoad>
                    </ext:Panel>
                    <ext:Panel ID="Panel2" runat="server" Title="Another tab" />
                </Items>
            </ext:TabPanel>
        </Items>
    </ext:Panel>
</body>
</html>
HomeTab
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>

<ext:Panel ID="homeTabLayoutWrapper" runat="server" Layout="column">
    <LayoutConfig>
        <ext:ColumnLayoutConfig FitHeight="false" />        
    </LayoutConfig>
    <Items>
        <ext:Panel
            ID="homeGrid"
            runat="server"
            ColumnWidth="1"
            AutoHeight="true">
            <AutoLoad Url="/Test/HomeGrid" ShowMask="true">
                <Params>
                    <ext:Parameter
                        Name="containerId"
                        Value="function () { 
                            return #{homeGrid}.id; 
                        }" Mode="Raw" />
                </Params>
            </AutoLoad>
        </ext:Panel>
        <ext:Panel ID="Panel1" runat="server" Width="200" Layout="accordion">
            <Items>
                <ext:Panel ID="Panel2" runat="server" Title="Events" Html="Events" />
                <ext:Panel ID="Panel3" runat="server" Title="Photos" Html="Photos" />
            </Items>
        </ext:Panel>
    </Items>
</ext:Panel>
HomeGrid
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
 
<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!X.IsAjaxRequest)
        {
            Store store = this.GridPanel1.GetStore();
            store.DataSource = new object[] 
            { 
                new object[] {"test1"},
                new object[] {"test2"},
                new object[] {"test3"},
                new object[] {"test4"},
                new object[] {"test5"},
                new object[] {"test6"},
                new object[] {"test7"},
                new object[] {"test8"},
                new object[] {"test9"}
            };
            store.DataBind();
        }
    }
</script>
 
<ext:GridPanel ID="GridPanel1" runat="server" AutoHeight="true">
    <Store>
        <ext:Store ID="Store1" runat="server">
            <Reader>
                <ext:ArrayReader>
                    <Fields>
                        <ext:RecordField Name="test" />
                    </Fields>
                </ext:ArrayReader>
            </Reader>
        </ext:Store>
    </Store>
    <ColumnModel ID="ColumnModel1" runat="server">
        <Columns>
            <ext:Column Header="Test" DataIndex="test" />
        </Columns>
    </ColumnModel>
    <BottomBar>
        <ext:PagingToolbar ID="PagingToolbar1" runat="server" PageSize="3" />
    </BottomBar>
</ext:GridPanel>
Controller
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Ajax;
using Ext.Net.MVC.Demo.Models;

namespace Ext.Net.MVC.Demo.Controllers
{
    public class TestController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult HomeTab(string containerId)
        {
            Ext.Net.MVC.PartialViewResult pr = new Ext.Net.MVC.PartialViewResult(containerId, RenderMode.AddTo);
            pr.ControlToRender = "homeTabLayoutWrapper";
            return pr;
        }

        public ActionResult HomeGrid(string containerId)
        {
            Ext.Net.MVC.PartialViewResult pr = new Ext.Net.MVC.PartialViewResult(containerId, RenderMode.AddTo);
            pr.SingleControl = true; //alternative of .ControlToRender if there is only one top control.
            return pr;
        }
    }
}