The Cologne Codex Mani (Cologne Mani-Codex, Codex Manichaicus Coloniensis) is the smallest codex known from antiquity. It is a vellum codex describing the life of Mani, the prophet and founder of Manichaeism. Written in Greek on leaves measuring 3.5 x 4.5 cm or 1.4 x 1.18 inches, and found near Asyut (the ancient Lycopolis), Egypt, it was originally the size of a small matchbox.
In December 2013 color images of the codex were available from the University of Cologne at this link.
See Henrichs, "The Cologne Mani Codex Reconsidered," Harvard Studies in Classical Philology, Vol. 83 (1979) 339-367.