You wont be able to force it to the full available width in the side menu div since it is in an LI tag, but you can increase the width by editing the #sideMenu ul a, #sideMenu ul a:link selector in the content.css file.
Add a display: block; and width property:
#sideMenu ul a, #sideMenu ul a:link {
font-size: 12px;
font-weight: bold;
color: #2d6191;
text-decoration: none;
display: block;
width:150px;
}