in the CSSFormBuilder/FD_Sharp_Default.css file
change the width attribute of the form.Sharp_Default textarea.formTextarea_Medium to 300px
form.Sharp_Default textarea.formTextarea_Medium {
background-color:#fff;
color:#000;
padding:5px 5px 5px 5px;
font-style:italic;
width:300px;
height:150px;
white-space:pre;
overflow:auto;
margin:0;
border-style:solid;
border-width:1px;
margin-left:2px;
border-color:#8c8c8c;
}