Thanx, era, for giving me a little hint, finally I found it out:
The "cols"-argument does nothing, as the width is specified in the style.css of Jcomments.
So it can be deleted from the .tpl-file, it has no sense (in my opinion, maybe something brwoser-specific, but I don't think so).
In style.css there is a passage:
#comments-form textarea {
color : #444;
background-color: inherit;
font : bold 0.9em Verdana, Arial, Sans-Serif;
max-width : 450px !important;
width : 400px;
padding : 2px;
margin : 0;
border : 1px solid #ccc;
Change max-width to whatever the hell you like
