What is your definition of "small"? New or used? Stock or modded? Any finish requirements or WAF factors?
Based on mostly reading alone, and without verifying prices, here are a few of the smallest speakers that I can think of off the top of my head:
AV123 X-LS
Usher 520
RAD Micros or Metals
Swan M1
Gallo Acoustics Nucleus Micro, A'Diva or Due
Good luck!