Assuming you're using Safari 3, Command-0 will "Make Text Normal Size". The same is true of Camino and Firefox.
iCab and Opera are hardcore, insist on actual Command-+ to make things larger (which is to say Command-Shift-=). Command-= (or Command-+ as the other browsers would have it...) is therefore used to revert to "Standard" in iCab and Opera offers a "100%" option under the View>Zoom menu, no keyboard shortcut.
Internet Explorer, a long time dead, nor the very much alive Omniweb, offer any way of getting back the standard font size other than manually opening the current page in a fresh window.