While I don't know of a way to change the type size in the Menu Bar, you can do these three things.
(1) Go to FINDER>VIEW>SHOW VIEW OPTIONS. You should see a drop down option to change the type size (from 10 to 16 point). NOTE: If you do this after clicking on the desktop, only the desktop type size will change. But, if you highlight your home folder (in the sidebar) first, then go to Finder>View>Show View Options, the change you make in the drop down option will change the type size for all of your fiinder views (except the Menu Bar).
(2) Go to SYSTEM PREFERENCES>UNIVERSAL ACCESS>SEEING. There you can turn on the Mac's ZOOM feature. Click on OPTIONS (I have mine set to Maximum Magnification 2x?Minimum Zoom 0. I also check marked "When zoomed in screen moves-Only when the pointer reaches an edge.") Click Done.
Then when you hold down both the OPTION & APPLE keys and press the plus or minus sign on your keyboard, you can zoom in and move your mouse pointer to any part of your screen (including the Menu Bar) and move in and out in it at will.
(3) If your mouse has a Scroll Wheel, you can also go to the SYSTEM PREFERENCES>KEYBOARD & MOUSE>MOUSE and check mark "Zoom using scroll wheel while holding..." At the same time, you can set the Option to "Zoom the screen only when the pointer moves" as well.
I hope this helps.