The largest chicken egg ever laid weighed a pound and had a double yolk and shell

The US Department of Agriculture grades eggs by the interior quality of the egg (see Haugh unit) and the appearance and condition of the egg shell. Eggs of any quality grade may differ in weight (size).

U.S. Grade AA
Eggs have whites that are thick and firm; yolks that are high, round, and practically free from defects; and clean, unbroken shells.
Grade AA and Grade A eggs are best for frying and poaching, where appearance is important.
U.S. Grade A
Eggs have characteristics of Grade AA eggs except the whites are “reasonably” firm.
This is the quality most often sold in stores.
U.S. Grade B
Eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken, but may show slight stains.
This quality is seldom found in retail stores because they are usually used to make liquid, frozen, and dried egg products, as well as other egg-containing products.
In Australia and the European Union, eggs are graded by the hen farming method, free range, battery caged, etc.

Chicken eggs are also graded by size for the purpose of sales.