What is the approximate accuracy of ASDE-X?
The FAA has a bunch of information on ASDE-X, most of which I personally have no idea how to interpret correctly. The ASDE-X procurement specification in particular has a lot of technical details on radar performance and although I didn't understand much of it I did notice this requirement:


  3.6.3   Map Registration 
  
  The display of target data shall [R306] be registered to the display map with an accuracy of at least 20 feet.


So whatever the technical capabilities of the system are, operationally it seems that you can assume it's accurate to within 20ft, which seems to be reasonable in the context of large aircraft. You may be able to get more out of the documentation than I could, of course.