If you are using the Thesis theme then all you have to do is add the following bit of CSS to your custom CSS file and that should do the job:
.custom .format_text input {
border-width:0.067em;
padding:0.2em;
width: auto;
}This page has more explanation on it:
http://www.tipsandtricks-hq.com/ecommerce/wordpress-estore-troubleshooting-218