True seals

The largest seal is the Southern elephant seal, also known as the Mirounga leonina.

An excerpt from the article 18 facts about True seals
It lives in Antarctic waters and reaches an incredible size. The average body length of the Southern elephant seal is 4.5 meters, while record-breaking individuals have reached up to 5.8 meters in length. The weight of these giants ranges from 1,500 to 3,000 kg, and record individuals reach up to 3,700 kg in body weight. They feed mainly on cephalopods and supplement this diet with fish.