The largest egg from a living animal is the ostrich egg, at 5 lbs / 2.3 kilograms (about the equivalent of 24 chicken eggs).
Ostrich eggs link Bird information
The largest egg from an extinct animal most likely would be from a dinosaur. However, only a few dinosaur eggs
(about 4 as of 1996) have been identified with the type of dinosaur it came from. One dinosaur egg was 18 inches long,
but rather thin.
Here is a National Geographic site about dinosaur
eggs.