In your menu.css stylesheet (CSSMenuWriter/cssmw/menu.css) find the line:
ul#cssmw0 > li > span > a {
and change the property for width from 'auto' to a fixed width for the level-1 menu items:
ul#cssmw ul.level-1 > li > a {
background-color: #999999;
background-image: none;
border-bottom: solid 2px #fff;
border-right: solid 1px #fff;
color: #ffffff;
display: block;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
padding: 5px 5px 5px 5px;
text-decoration: none;
width: 140px;
}
Also add the width to:
ul#cssmw ul.level-1 {
left: 0px;
top: 100%;
width:140px;
}
IE needs widths to be specied. You can adjust the width to suit your needs.