Pete, I'm certainly no expert...but IMO not less than 300 gallons for even a small species.
I've thought alot about it and I'm absolutely against people keeping sharks. I have to admit, I'm starting to lean toward the position that we shouldn't be keeping fish at all. Sure, they're beautiful, but you're taking a living creature out of it's environment and putting it into a cage for no reason other than you're own amusement.
Regardless, I view sharks as a higher species. If you want to capture a shark and keep it on display, please at least give it the courtesy of a decent home.
FTC