Okay - you have told us what the width should be... any reason you haven't told your css ?

Tell you Nav Container the width you want!
Additionally, you may want to wrappers/contaiers for the nav... once with the full width, one that "shrinks" around the content... then center the shrunk one.
As it's parent has a defined width, it should beable to calcualte a position from there!
please think twice before applying a set height for the nav - as text-resizing will cause the text to break out!
Further, what happens if the text gets bigger in such a short widht.... it could result in the links dropping to two lines not one... in which case it will break your design due to fixed height!
Basically... set widths, not heights!!!