Would this one become the world smallest harmonica? This Hohner harmonica has a dimension of 1.375×0.375×0.5-inch (a little bigger than a rounded shape paper clip).

world-smallest-harmonica

And this Hohner harmonica is the smallest harmonica sold over the VAT19.com. It cost a mere USD14.95 and has the capability to play a full octave in C-Major. It comes with a key ring attachment.

Considering the size, you could simply play using a single hand or two fingers if you are an expert.