The Titan Computer EZ Sound 2.1 Channel Speaker is supposed to be the world's smallest.
The Titan PT-100 series measures 113x94x45.9mm and the two satellite speakers have each 0.25W output. The integrated subwoofer has 1.5W output. The supported frequency spectrum is 200Hz~18kHz.
DragonsteelMods reviewed the Titan EZ Sound system and gives it a thumbs up. Read the full review.
See also the Titan EZ Sound product page. No pricing or shopping source available.