You need to adjust the table width. Take a look at a test html template code and look for the code near the top of the page. It'll look similar to this
<body>
<table style="width: 100%; background-color:#ffffff;" border="0" cellpadding="0" cellspacing="0">
<tr>
<td style="vertical-align: top; padding: 10px" align="center" valign="top">
<table style="width: 660px;text-align: left" border="0" cellpadding="0" cellspacing="0">
If you change 660px to something smaller, that should work. No guarantees though.