in the admin/css/admin_settings.css file edit:
#addTaxRule, #addShipRule {
-moz-border-radius:5px 5px 5px 5px;
background-image:none;
left:50%;
margin-left:-350px;
top: 50px;
padding:30px;
width:600px;
}
to:
#addTaxRule, #addShipRule {
-moz-border-radius:5px 5px 5px 5px;
background-image:none;
left:50%;
margin-left:-350px;
top: 50px;
padding:30px;
width:600px;
height: 200px;
overflow: scroll;
}