I just recently purchased this Logitech Mini Boombox (new version is the Logitech UE Mobile Boom Box) for my iPhone and iPad. It is amazing! It is about the footprint of your iPhone and about 3 inches tall. It comes with a rechargeable battery that lasts up to 10 hours. The speaker can also be used as a handsfree calling device to receive calls. The controls are all touch sensitive controls on the top of the speaker. You can play/pause, skip forward/backwards, and receive a call. The sound is great and packs more bass than you would expect from such a small form factor.
Here is the link to buy the speaker directly from Amazon: