No. 0 Lever Filler
In terms of size, Collectible Stars I shows examples of lever fillers in OO, O, 1, 2, 4, and 6. The size O measures only about 10 cm. That is tiny, even though it's not the tiniest. See how it compares to my celluloid 149 and a No. 1 Ret and No. 2 safety in the picture.
Rösler, J., & Wallrafen, S. (2014). Collectible Stars I: 1908 - 1947.