I'd recommend heading down to a stockist and trying one out before getting the biggest you can find, because the entire tablet represents your monitor a larger tablet means you have to make much bigger hand movements to achieve the same thing on a smaller one. I find the 6x8 is a nice size and there's not too many times I've wanted a larger one. I'll probably upgrade when I get bigger monitors though
EDIT - If you're running dual screens, get the intuos (unless the bamboo supports them properly now?)