Well, really, it's not quite so simple - it depends on what version of JQM you are using, and just what you mean by "fixed" toolbar. And, honestly, I don't recommend monkeying with the page width, as it will cause problems with MOST transitions. If you insist, I suggest you use transition "none". I've seen others try this, and have never found the result pleasing, but you have to be the judge.
I'm assuming you are doing this to keep it from looking silly on desktop. I'd rather you just encourage desktop users to adjust the size of their window. Which I think they will on their own, since it will, indeed, look silly if the page is too wide.
- You do not need any code for this in any case. Just use responsive CSS rather than using code. It will be a smoother result.
The ideal situation is "fixed external toolbar". This is completely outside of the page. It takes a bit of code to set this up.
If you define your toolbar inside of the page, then you have the option of making it a "persistent" toolbar. In older versions (not sure about 1.4.5) you could set the same data-id on the toolbar in each page, and it would make it persistent across pages. However, styling it is problematical, as if you examine the code and behavior, you will see that the toolbar actually is moved out of the page during transitions, and then back into the page once the transition is done! It is a mess! So, you need to make sure any styling changes accommodate the toolbar both in and out of the page (you will need two selectors for your CSS).
There may also be an issue with having sufficient specificity in your CSS selector. Your desktop browser's developer tools will help you work-out the specific selectors needed. You can get around this with !important, but better to work-out the specificity.
If you still have trouble, I can look at some OLD code of mine that changes header/footer opacity and height (not width) and see just what I did. But I think I have this only for 1.2.
(Edit: it appears that a selector of .ui-header-fixed (for header) or .ui-footer-fixed (for footer) is sufficient, at least for 1.2, probably as well for 1.4.5).
I really do not recommend monkeying with the width of either pages or header/footer. JQM was designed to take the entire width of the window, and the results are not likely to be pleasing if you change it.