Open up header.php. On line 298, you will find:
<div id="logo" class="span3 clear fix">
On line 307, you will find:
<div id="nav-section" class="<?php echo $nav_class; ?> span9 clear fix">
You will notice the span3, and span9. These relate to the grid widths in the Twitter Bootstrap framework. These must add up to 12, so you can change these to match the widths you need. Below is a list of the span and their width at the largest size (1200px+)
span1 - 70px
span2 - 170px
span3 - 270px
span4 - 370px
span5 - 470px
span6 - 570px
span7 - 670px
span8 - 770px
span9 - 870px
span10 - 970px
span11 - 1070px
span12 - 1170px