Get one, get one, get one
I have a 12 x 18 Wacom tablet that I use with Illustrator and Photoshop on a regular basis. I agree with Jeff that I have never used all that space and, at times, it is way too much.
My business partner has a 6 x8?, smaller anyway, and uses it constantly. If she can use it instead of the mouse, she does. We do not have the 4-D mouse but find the pen very accurate and much easier to use to pinpoint the points on the paths within Illustrator. It is so much easier to work with the detail within enlarged images using the tablet that we cannot image doing without one.
One of the neat things with a Wacom tablet is that you can program certain functions to correspond with routine Photoshop and Illustrator activities. This may also be true with other tablets or programs. Just one more thing to let you focus on the fun stuff instead of reacting to hardware/software limitaions.
For what it is worth, coming from a guy, do not do the guy thing and think bigger is better. Unless you are working with CAD or architectural type images I do not see much point in using a bigger tablet. Besides the fact that a smaller one leaves more room for your coffee or cat.