iHome iH19

The iHome iH19 measures only 9 x 4.3 x 3.5 inches, and its integrated case (in pink, white, or black) is splash resistant and unzips, so you can nestle your iPod in the interior's shock-absorbing foam. The sound quality isn't bad for the size, though if you crank the volume (it doesn't get very loud), the bass distorts.
 
The iH19 comes with a locking mount system, so you can attach the speaker to, say, your bicycle and remove it easily. It also runs on four AA batteries or AC power and can charge your iPod as it plays, using the built-in dock-connector cable.