you will find .right – which is one of the two classes being applied to your nav. Strangely though, its already set to 550px wide. Try changing it in some other way, like margin or whatever to see if that has any effect on the whole menu.
Normally I would have wrapped the whole navigation in a #div of its own and floated it to the right. If that was set to 550px the menu inside would take up as much space as it could.