For that size of screen you're looking at an A5 wide (27x15cm) as a minimum. Otherwise, you're going to find it somewhat imprecise, as the pen will use the active area as if it were the entire screen.
I don't know if there's a suitable Bamboo model so you might be looking at an Intuos 3 A5 wide instead. I could only find a wide version of the A6, which is too small.
I've actually been considering the Bamboo Fun as well. I've looked around and researched a lot, and from what I've heard, the size doesn't seem to take away too much from it.
Try and find some people who have bought it. See what google turns up. Pro reviews would be good too, but I wouldn't rely on just one review. It's pretty new, so it's hard to tell what the longevity is, but at least you've got a 2 year guarantee.
I currently have a 22Inch Widescreen LCD, I was wondering will the Small be to small?
I want peoples opinions that Have used them, thanks.