To start with, the SprymenuBar is not suited for touch screens, it will not work.
You are better off using a jQuery or a commercial menu.
Even then, you must provide the styling rules for the different widths. Fixed widths, such as those in the SpryMenuBar are a no no.
Have a look here http://www.projectseven.com/products/templates/pagepacks/css-builder2/index.htm to see what happens when you go to narrower screens sizes or here http://foundation.zurb.com/page-templates/homepage-1.html. The former is a commercial product, the latter is free of charge.