It depends on where you plan on typing.
If it is mainly at a desk, I can only recommend the Apple Bluetooth keyboard.
But if move around a lot and want the smallest footprint possible, look for some foldable bluetooth keyboards. The keys are not much fun to type on, but if the only other choice would be no keyboard at all, it may be worth it.
Personally I find the Apple keyboard somewhat large, especially when compared to the iPad 2. But it is the smallest full keyboard with keys you can type on for hours. Price is around 60 pounds.