Smallest body that can be observed from Earth? What is the minimum size of an object that can be observed from Earth with our most powerful telescope? 

I understand that the answer depends on how far the object is from Earth. For the sake of  discussions let us consider distances less the orbit of Neptune.
The smallest natural object that I know of that has been detected from an Earth-bound telescope is 2011 CQ1, a one-meter asteroid observed while specifically looking for NEOs.

As DeerHunter mentions in a comment, smaller artificial objects are identified and tracked, mostly by NORAD and some other national defence centers.