This is a web browser issue, not an Axure issue. Axure has no method to scale HTML used in preview or generated when requested.
As the "Menu" (by which I presume you mean the Axure controls) is also HTML, it will scale when you scale the HTML view within the browser.
The only suggestion I can think of is to use the dev tools to manually override the "menu" HTML. If you use firebug (via firefox) or chrome, then just right mouse click on the menu items, find the CSS and override the size. This will let you see the "Menu".
I very rarely use the "Menu", I'm curious as to why you use it.
If you use it to navigate between pages in your prototype then I would suggest that your prototype use controls to let you navigate between the pages. If you do not want this then you could add hidden controls (hot spots). You could also just edit the URL in the browser to the page that you want. The final option is that you launch the preview with the page open inside Axure that you want to see, it will always load based on the context of which one you have active in the editor.
If you use the "menu" for the console (debugging events), then you will have to resize the "Menu" as per above.