Well the way that you have it worded, you want it to fill the entire nav at the top, so you want, this will change the spacing between each of the list items to the left, you can also add padding-right
to change the amount of spacing between the right of each item
#cssmenu ul li {
float: left;
display: block;
padding: 0;
padding-left:13%; /* Change this value to fit your screen */
}