If you are a Tabbed browsing lover and want more space for your Tabs then you can make the Address Bar smaller. Moreover, if you prefer to have a larger Address Bar, then you can increase the size of Address Bar in Internet Explorer 9 as per your wish.
I've been using IE9 Beta since its release and have mentally whined about the size of the address bar (i.e. wishing it was a little bigger). Low and behold the power to easily and quickly resize it has just been a click and drag away this whole time.
It would be ironic if this went away in the upcoming, week after next so I've read, IE9 RC wouldn't it? lol