i don't know about the smallest pl*c in existence, but as far as the species that are very common in the aquarium trade....
clown pl*co (3" according to most sources)
bristlenose pl*co (5-6")
rubberlip pl*co (also called bulldog, rubber-nose. despite what the info cards at petsmart say, this is a dwarf species which should top out around 6")
there are plenty of other uncommon and/or dwarf species available as well, but buying them is often an expensive venture.