What is the largest tetra?

That depends on where you draw the line between what is a Tetra and what is not. Because "Tetra" is not taxonomically defined (like Cichlid=Cichlidae), it is highly variable. Fish like Hydrolycus scomberoides can be considered a Tetra, at 2.5 feet in length. Silver Dollar species are also often referred to as Tetras, as are Piranha. In a stricter definition, though, Exodon paradoxus is probably the largest, at nearly 5". Phenacogrammus interruptus is a close second.