There are several factors that influence how small a piece of debris a radar can detect, including frequency, antenna gain, uplink power, receiver sensitivity, etc. The military space surveillance radars that I have experience with (as an operator or program manager) normally can't track objects smaller than -30 dBsm (10 cm) RCS. The Haystack Aux radar was designed to do statistical sampling on object as small as 1 cm plus some other stuff that I'd rather not talk about.

If you're talking about millimeter sized objects, existing radars don't work very well. When you go to frequencies high enough to track stuff that small, you tend to run into problems with atmospheric absorption of RF energy due to water vapor. Optical sensors can be made to track really small stuff in LEO but with some severe limitations. For example, you need clear skies and can only work for roughly an hour or so after sunset and before sunrise. There is another possibility that might work - using a LASER "fence" to provide illumination at other times. That idea has some potential problems that must be addressed, such as not blinding pilots who might fly in the area. LASER illumination and ranging has been used for certain things but I don't know if it has ever been used for space debris studies.

NAVSPASUR is a good sensor but it currently operates on too low a frequency to track really small debris. I've heard there are some upgrades in the works that should improve that. Here's a pretty good overview of space surveillance systems currently in operation.