For the tinymce wysiwyg editor at least, you can use CSS to set the width of the editor. Just note that you can't set the width less than what it needs to fit all the icons. This is also a valid HTML5 solution as opposed to setting the width using the 'cols' attribute.
CSS:
textarea.wysiwyg {
width: 450px;
height: 200px;
}
HTML:
<textarea name="whatever" id="whatever" class="wysiwyg"></textarea>