Smallest common size is the flat bottom 5/8" x 3/8" shank version taking 1/4" toolbits. Nominal minimum tooth height above the base is 1/2" with nominal 1/8" (ish) projection. I have examples badged Armstrong / Denford / Andycraft / Homecraft in the "why are you keeping those" box. All subtly different but close enough that you need to read the name to see which one you have grabbed. J&S may have made a similar size with their typical dropped front design. I imagine the shank would have been 1/2" x 3/8" putting the tooltip about 1/8" lower. Obviously needs to go in a block, 4 way or lantern type toolpost to accommodate the drop but flat bottom ones can go direct on the topside. I want to say that I've had J&S types in that size through my hands and let them go with my SouthBend Heavy 10 for which I made multiple 4 way toolposts to use as a poor boys QC system but bottom line is I can't recall exactly what I had.
There is a 1/2" x 1/4" nominal shank size version taking 3/16" toolkits. Seems to be very rare as I've only ever seen the one which came with my first lathe, a Portass S. Disappeared sometime in the intervening 40 + years but I suspect it was made by Eclipse to match their smallest parting blade holder, 2 of which I still have.
Clive.