go to templates->header.tpl and find line
</head>
<body>
<table width="780" border="0" align="center" cellpadding="0" cellspacing="0" bgcolor="#FFFFFF">
<tr>
<td>
<table width="780" border="0" align="center" cellpadding="0" cellspacing="0" bgcolor="#FFFFFF">
<tr>

just change 780 to 1000