Specific historical cittern measurements can be found on subpages of the Musikinstrumenten-Museum der Universität Leipzig Katalogprojekte, http://www.studia-instrumentorum.de. (Site in German). See especially:
Four Freiberg citterns. Drawings available directly from the University of Leipzig. Contact Prof. Dr. Eszter Fontana
Cittern by A. Citaraedus, Urbino 1582. Drawing by Stephen Barber. Available from the Victoria & Albert Museum. No. 392-1871. [Note: As of Jan. 1, 2008, it was not possible to find the drawing listed on-line, but the prints department has made assurances that it can still be ordered.]