It used to be that you could find a Magnify item in the Format menu of the Mathematica front-end, at the very bottom if I remember correctly. You could get at the item in Windows by a keyboard sequence like
M, then a number. Mac probably had something similar.
This was really useful for mouse avoiders like me. That little tiny drop menu in the extreme lower right of a notebook is annoying to catch with a mouse on highest-speed setting, which you need on a high-def screen with 1920 or 2560 pixels width. My mouse movements end up looking like a plot of a strange attractor swirling around the target!
I haven't been able to find the Magnify menu item in Mathematica 8 nor any other keyboard shortcut for notebook-magnify in the Mathematica documentation center. Would be grateful for anyone pointing out any means for accessing notebook magnify from the keyboard.