What is the smallest aircraft to have a CVR / FDR?
Looks like in the EU as well as in the US the Cessna CE-510 Mustang needs one according to the minimum equipment list.  

In the EU 


  -70-01
  
  *** Cockpit Voice Recorder (For Combined Voice Data Recorder, refer to 31-30-01)


And in the US 


  -30-01
  
  *** Flight Data Recorder (FDR) System


That is a 6 seat plane, you will be hard pressed to find something smaller although other similar in class aircraft may also have them.  

It looks like some versions of the Cirrus SR-22 include an Aircraft Data Logger but im not sure if that conforms to the various impact regulations for a black box.