The smallest and lightest is probably the Tele Rokkor 11cm f5.6 (made by Chiyoko, later known as Minolta). It is shorter than most 85mm lenses and, consistent with its speed, very thin.
The smallest you are likely to find, and great to boot, is the aforementioned Canon 100mm f3.5. The older chrome/black is narrower than the later all-black, but both are small and light. Watch out for internal haze and fogging, though.
Cheers,
David