Styling the comments will happen in your skins/your_skin/stylesheet.css file (whatever it happens to be named). The structure of the comments is defined in one of two places. It might be skins/_feedback.php, and it might be skins/your_skin/_feedback.php. If you don't have that file in your 'your_skin' folder you can copy it from skins to skins/your_skin and edit it there. Either way though, that's the file that you want to edit to make structural changes to your comments section.

Thanks, I think that will help...

Ok, didn't get it... which exact entry in my custom.css I have to modify to make the comment boxes a little bit wider?

I found this and changed the 97% to 99% and even to 100% but nothing happens... Is this the wrong entry? Which is the right one?

input.bComment,
textarea.bComment
{
	background-color: #eee;
	border: 1px solid #ccc;
	width: 97%;
	padding: 2px;
	margin: 0ex;
}

In your CSS files find "comments.css"
Change this bit from...

div.bComment, /* Entire comment block */
form.bComment /* New comment form */
{
	margin: 1ex 2ex;
	padding: 0 1ex;
	border: 1px dotted #999;
}

to

div.bComment, /* Entire comment block */
form.bComment /* New comment form */
{
	margin: 1ex 0;
	padding: 0 0.5ex;
	border: 1px dotted #999;
}