Okay, I'm going to assume you're talking about home aquaria.
The smallest species of shark that you can get for home aquaria - is the Izu Catshark - it's a cool water species(water temps about 60F) from Japan, and it only grows to about 16 inches (41cm) in length. But these are fairly hard to find and fairly pricey -if you do find them.
The smallest tropical or warm water species - is the Australia Marbled Catshark - which is found in shallow tidal pools, of northern Australia.These are much easier to find and grow to about 24inches (61cm).
This is a quote from one of our members here krj,
As for tank sizes probable if you go with this catshark (Izu) it will thrive in the 210 but probably a standard 300 gallon best....all about dimensions/filteration