tunalegs
Pretended Artist
End to end the Bolsey B might be the smallest full frame 35mm rangefinder. But only in that particular dimension.
rwintle
Scientist by day
Not quite the smallest but very similar in size to the XA2 mentioned above is the Voigtländer VF-101 at 102 x 73 x 35mm - presumably the Zeiss Ikon Contessa S 312 is the same(ish).