if you want to change the width’s find the following bits in style.css in that themes directory. They’re pretty near the top.

These lines control the width of the posts area

div#wrapper{clear:both;
width: 485px;
float:left;
}

This controls main navigation width:

div#navigation{
float:left;
width: 280px;
margin-bottom: 1em;
}

This controls the ‘header’ navigation width

div#extra-sidebar{
height:285px;
width:285px;
float:right;
text-align:left;
background: url(images/extrabar.jpg) repeat-x;
}

Now, if you want to increase the posts width by say 40px. Add that to the existing number. So you get 525px instead of 485px and then subtract it from the navigation widths so you get 240px and 245px.

That should do it.