I recently bought this:
http://www.play.com/Electronics/Electronics/4-/18814905/XMI-X-mini-Version-MAXV1-1-Dual-Capsule-Speakers/Product.html?searchtype=allproducts&searchsource=0&searchstring=x-mini&urlrefer=search
It's not Bluetooth, but is quite small with surprisingly big sound.
Another good option is this:
http://www.amazon.co.uk/Sony-SRSBTV25-Bluetooth-Wireless-SmartPhone/dp/B005U9XZLW/ref=sr_1_5?s=electronics&ie=UTF8&qid=1329847323&sr=1-5
Bear in mind, this is mains only operated, & not battery.
Bose has a brilliant Bluetooth speaker which is quite light. It definitely sounds the best Bluetooth speakers I've heard so far & has a rechargeable battery. Check it at your local Bose store to see if you're happy with its size.
http://www.bose.co.uk/GB/en/home-and-personal-audio/digital-music-solutions/bluetooth-speakers/soundlink-wireless-mobile-speaker/