Various factors affect the size of an egg. The first factor is the age and weight of the bird. Younger, lighter-weight birds tend to lay lighter, smaller eggs; on the other hand, older, heavier birds tend to lay heavier, larger eggs. Another factor is the breed of the bird. Differing genetics, makeup, and structure of the bird all affect the size of the egg. The third factor is the nutrition the chicken is receiving. The amount of protein, the quality of the grains, and sufficient water all play a part in affecting egg size. Finally, the size of an egg is affected by the weather. Too many hot days in a row result in smaller eggs. Even though there are many factors affecting egg size, the largest percentage of eggs that are laid are either large or extra-large.
Egg size is determined by the weight of an egg, not the circumference. Typically, there are 5 sizes of eggs: small, medium, large, extra large, and jumbo. Small eggs are 1.5 ounces per egg, medium eggs are 1.75 ounces per egg, large eggs are 2 ounces per egg, extra large eggs are 2.25 ounces per egg, and jumbo eggs are 2.5 ounces per egg.
Size does not affect the taste of the egg, so take your pick!