looking for smallest conventional speaker with decent sound

I have a home theatre setup and am looking for recommendations for the smallest conventional rear speakers. They are for 5.1 ambient sound so am not interested in speakers that are expensive but only want them to have low distortion and be small enough to be inconpicuous.
Has anyone come across tiny speakers that produce acceptable rear ambient sound.
Would appreciate any recommendations from any of you that have found cheap small speakers for this purpose.

Thanks guys for your input. I'm looking for a speaker ideally half the size of that BA 6" cube. If you have ever heard of such a small conventional inexpensive speaker, please let me know.

I have come across several references to tiny speakers that claim relatively decent performance, but they are usb powered designed to work on lower wattage ipad type feeds.
If there was such a speaker that could be fed by rear channel speaker outputs from an a/v receiver they would be ideal.

I currently use some old (cheap but good) Radioshack minimus speakers I bought in the early 80's and they are 7x7x4, but I am looking for a smaller form factor.