Setting the above code will work, with limitations of the dnn menu itself.  The width will be set, but if a single word is wider than that width then the width will "stretch" to fit, which is what you want it to do.  If it didn't stretch to fit, you'd have menu text outside the menu background and it would look awful.   Anyway, if a single word is stretching the width, all the menu items above it and below it will still have the same width.  I set the width all the time, but realize it is really not a fixed width, but more of a minimum width.  So, if you want all the menu items in all submenus to have exactly the same width, then find out the width of your longest word and set the width to that, then they will all be uniform no matter what level they are and no matter what root item they drop down from. 

If you are trying to use a fixed width background image, don't, the dnn nav menu can use them, but most of the time you'll run into this issue of menu text stretching the width or height and extending beyond the menu item's individual image.  But, again, you could work around that by making sure the image is the height and width of your tallest and widest menu item text.

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