Boy, that goes back a long way. As I remember, back in the 70's, when I bought my ME, it was the smallest SLR available, just a hair smaller than the Olympus OM-1. The ME measured 131 x 82.5 x 49.5 mm (no lens mounted). MX was the same I think. For reference, the Spotmatic F was 143 x 93 x 94 mm with a 50 mm lens mounted.
That said, I hunted around a bit (i.e., googled smallest screw mount SLR) and found the Fujica ST605N, which measures 133 x 86 x 88 mm with 50 mm f/2.2 mounted; the Fujica is quite close in size. The Fujica is 565 g, body only, while the ME is 460 g. The Fujica has a Praktica thread mount, which is the M42 mount screw mount. The MX with a screw mount adaptor would be a manual film SLR smaller than the Spotmatic and slightly smaller than the Fujica.
As a nearly full-service replier, I note that there are a number of the Fujicas available on...an auction site I checked. None of them are mine.
EDIT: dcshooter was first-in with the Fujica. I type too much.
EDIT 2: the Chinon CM-3 is 140.5 x 88 x 50 mm and 610 g.
Last edited by stens; 03-12-2016 at 09:55 AM.
Reason: updated for completeness