The Tiniest?

[tunalegs]

End to end the Bolsey B might be the smallest full frame 35mm rangefinder. But only in that particular dimension.

 

[rwintle]

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).