i dont know how small you can get a subwoofer, but i doubt a subwoofer any smaller than my altec lansing (more or less 1.5' x 1') wouldnt be worth buying. subwoofer work off of the physical principle of being able to produce very long wavelength sound (deep tones). it follows that physical SIZE is actually one of the leading determining factors of a subwoofers quality!
why dont you invest in a set of small, yet high quality, speakers?