The smallest is Hector's dolphin at about 1.5 meters and 60 kg. Their teeth are sharp and peg-like in shape and function for grabbing prey, not for biting out chunks or chewing. Dolphins can be aged by counting the growth rings in their teeth just like in a tree. Dolphins live in world of sound. They use sounds to communicate and each animal has its own signature whistle which acts like a name.
Dolphins do have a large brain and are superbly adapted to their environment.
In captivity dolphins have been taught a type of language and can understand complex instructions, just like a dog or horse. Dolphins live in such a different environment than ours that trying to compare intelligence to other species is not really useful.