I had the same issue, what i did was

Retrieved the parent element's width
subtracted some 35 (which suits my width)
set it to the width

var grid = new Ext.grid.GridPanel(
{
id : 'gridPane',
cm : cm,
store : store,
tools : tools,
title : gridTitle,
//autoWidth : true,
width : datagrid.getWindowWidth(),
. . . . .
. . . . .
. . . . .



Where getWindowWidth is

getWindowWidth:function(){
return window.parent.$('center').parentNode.style.width.split("p")[0] - 37;
}