The Norand Mono is 300x150x42 (about a foot by 6 inches and an inch and a half thick) so we’re not talking large. That’s why I asked earlier if the O.P really wanted the tiniest — or was he talking smallest as a class ?
I know this is digital but perhaps someone makes an analog equivalent in a MIDI plug which would be the smallest.
H-Pi Flash Synth World’s Smallest FM Synth. I think they were considering a virtual analog of this.