The largest group of all living things is the ant.
A county.
The largest animal is the blue whale.
Carbon mostly but also nitrogen makes up the largest part of living things.
county
Whales and sharks.
Christianity is the largest religion; Catholicism the largest subdivision.
The longest in geological time is an Eon eg. Archeozoic Eon . Edit: there is also the supereon, which is usually composed of 2-3 eons.
Kingdom
The Great Barrier Reef is the world's largest living organism.
The cecum is largest in herbivorous animals, such as herbivorous mammals like cows and horses. These animals have a larger cecum to help with the fermentation of plant material and the breakdown of cellulose.
The Giant Sequoia trees are the largest living things on earth.