Smallest commodity Li-Ion cell is the insanely small
10180, but the mAh rating is a joke (claimed 80, really about 50, or about 200 seconds worth of vaping). There's also a 10280, only half again the size but twice the capacity (still half that of the 10440). The names are sizes, the first two digits are diameter in millimeters, the second pair are length in mm, and the trailing zero indicates a cylindrical battery.
Smallest standard protected cell is the 16340 (aka CR123). 3.6V isn't a random number, it's the inherent voltage of Lithium Ion chemistry (and actually varies from 4.2V down to 3.0, above or below that you're damaging the cell, potentially to the point it bursts into flame). Lithium non-rechargeables have a 3.0V level. There's also a few other checmistries out there, like Li-Polymer (3.7V, also dangerous but higher energy density, mostly used in flatpacks), LiFePO4 (3.2V, but not vulnerable to turning into a firebomb) and LiMN (3.7V, also "safe", but 25% less energy capacity for the same volume).
That's about all you're going to find as a commodity cell available on the market, although you can run into some weird stuff built for specialty purposes and then leaked out through salvage or remainder sales. Saw D-cell sized 32620's the other day (like 10Ah rating each).
--Dave
yanked out of my factory 
