It depends how you want your stuff set up. I don't find it too small at all for a 24 inch monitor, not to mention wacoms can scale to your screen.
That being said if you plan on using it on both monitors at the same time, you may want to opt for a larger size since to map both screens to the tablet and keep the proper aspect ratio (so when you draw a circle you don't get an oval) it would leave you with some "dead space" top and bottom on the tablet.
That being said the new intuos has "display toggle" which I think will make you quite happy. If you want to switch screens and keep the full real estate of the tablet just click the display toggle button on the side of the tablet.
I myself LOVE having a wireless wacom. I use it so much more now since I don't have to keep it plugged in and fuss with a wire. I won't trade it for anything 😀