Sorry I can not help you. I do not understand the problem.
The user has the option to alter page width in the browser. So a menu that runs under a banner will change size if the user wants to zoom in or out or if he has a low resolution device.
For example, the Computer Hope banner and menu bar with adjust itself as the user wants until it has to word wrap the menu bar. The banner really has two parts.
To the left is a n image that with re size as needed
To the right is an image that will more off to the right as needed. It is a background
The only thing I can recommend is to examine the code of sites you like and see how they do it.