Depends on the tracking mechanisms employed. For example, using radar, the object you're tracking must be larger than the wavelength of the radar being used. Also note there is a difference between focused "tracking" and "searching".

Radar also suffers from atmospheric attenuation, so there's a tradeoff between frequency, power, wavelength, etc.

I don't know what systems they're using to track LEO objects, but it's certainly within the realm of reasonable.

A decent primer: http://faculty.nps.edu/jenn/Seminars/RadarFundamentals.pdf