Several factors influence the size of an egg. The major factor is the age of the hen. As the hen ages, her eggs increase in size.
The breed of hen from which the egg comes is a second factor. Weight of the bird is another. Pullets significantly underweight at sexual maturity will produce small eggs.
Environmental factors that lower egg weights are heat, stress, overcrowding and poor nutrition.
All of these variables are of great importance to the egg producer. Even a slight shift in egg weight influences size classification and size is one of the factors considered when eggs are priced. Careful flock management benefits both the hens and the producer.
Egg sizes are Jumbo, Extra Large, Large, Medium, Small and Peewee. Medium, Large and Extra Large are the sizes most commonly available because hens most often lay eggs of these sizes. Sizes are classified according to minimum net weight expressed in ounces per dozen.
Which Size to Buy
You can use any size egg for most basic egg recipes, including scrambled or fried eggs. However, most recipes for baked goods are formulas in which it’s important to maintain the proper proportion of liquid to dry ingredients and to have enough whole egg, white or yolk to perform the needed functions. Most baking recipes are based on Large-sized eggs. (To substitute one size egg for another in recipes, see Size Equivalent.)
Although you can use any size egg for frying, scrambling, cooking in the shell or poaching, most recipes for baked items such as custards and cakes are based on the use of Large eggs.
To substitute another size, use the following chart.
You can also figure how many of which size to use by cup measurement. To make 1 cup:
If you have ever been in a quandary over what to do when the recipe calls for a large egg and all you have are jumbo, I hope this post helps you out.
Tomorrow I will get right the the middle of things and talk about using the yolk. Look for a delicious recipe as well.
Adapted from: http://www.incredibleegg.org/egg-facts/eggcyclopedia/g/grading