I can't really comment on comparing different models and such because I've really only used the 8x6 Wacom Intuos. I have the Intuos 3, but I've used the 1 and 2 while in college (all 8x6). I really like those.
You mentioned "using your whole arm", and I'm not quite sure what you mean by that. But I have heard of a friend of a friend (so take it for what you will) that likes to use their whole arm to make big sweeping motions while drawing. And so they purchased as big of a tablet as they could get. I personally think that's overkill. And while I've never used one of those really large ones, I think I prefer the 8x6 size because it gives me plenty of drawing room while at the same time it fits into most laptop bags wonderfully. Basically, if you are comfortable drawing on a 8x6 pad of paper, then that's probably a fine size to get.
Also, I should mention that if you are a student, you may be able to get one at a discount. That's how I got mine.