I have the really big (don't remember the exact size, but it was the largest Intuos II) on my desktop. I use half of it for each monitor.
I have the Cintiq 21 for the laptop.
As neat as the monitor and tablet together is, if I had the choice today, I would go with the Intuos III over the Cintiq for portable use. Too many cables and such on the Cintiq to make it convenient.
If pressed, I'd say bigger is better, because if you decide you need a smaller work space you can actually use the tablet software to map the tablet to whatever dimensions you like.