My Hubba (2006 model) does not get wider than the floor as it goes up. However, it doesn't get much narrower - the cross pole really does a good job of making the walls about as vertical as they can be. I don't have mine set up (and am leaving on vacation in the next couple days, so won't have time to), but I've never had any trouble sitting up in it and not brushing my shoulders agains the wall. My best guess would be that it's about 20 inches wide at the top, maybe 21 or 22 at shoulder height. (The floor is about 26 inches wide, staked out.)

I don't know if they made any changes to this in the 2008 revision - that's a Jason question.

Did I mention I love my Hubba? (And that's after trying out a lot of other tents.)