This is easy, simply go to "/lib/tpl/monobook/monobook/main.css" then search for these lines:
.portlet {
border: none;
margin: 0 0 .5em;
padding: 0;
float: none;
width: 11.6em;
overflow: hidden;
Here you can change these line:
width: 11.6em;
Now you must change the body width:
#content {
margin: 2.8em 0 0 12.2em;
padding: 0 1em 1.5em 1em;
background: white;
color: black;
border: 1px solid #aaa;
border-right: none;
line-height: 1.5em;
position: relative;
z-index: 2;
}
Here you must change this line:
margin: 2.8em 0 0 12.2em;
Make "12,2" higher than the width of the previous edit.
Now save an enjoy the new width of the navi side bar.
Martin