Chicken eggs fall into six official sizes, from peewee eggs weighing about an ounce to jumbo eggs weighing 2 ounces or more. If you're wanting to maximize egg size in your flock, a number of variables will come into play, including breed, age and diet.
Check Her Breeding
Generally speaking, the larger the adult hen is the larger the eggs she'll lay. Full-size breeds such as Rhode Island Reds, Plymouth Rocks and Buff Orpingtons will lay larger eggs than Silkies or other bantam breeds. The largest egg on record was nearly 12 ounces and measured 12.2 inches around. It was laid by a Black Minorca in England. In the United States, a white leghorn laid the world's heaviest egg at 16 ounces.
Wait for It
Just because a hen is from a large breed doesn't mean she'll start laying big eggs right away. Immature female chickens or pullets start laying their first tiny eggs when they are 16 to 22 weeks old. Her first eggs are peewee size as her reproductive tract gets used to laying. Her eggs will become medium to large by the time she is a year old, and increase in size older she gets. By the time she is 2 or 3 years old, she will have decreased egg production, but her eggs may be extra-large or jumbo in size.
The Role of Nutrition
A bird's weight factors into her egg size. Sufficient protein and essential fatty acids allow a chicken to develop to the maximum size for her breed. Scrimping on food costs results in smaller eggs. Letting her forage on fresh greens and insects in your yard is one way to supplement her layer feed inexpensively while giving her optimal nutrition.
An Earthy Supplement
Supplementing your hen's diet with diatomaceous earth can not only increase her weight and egg production, but egg size as well. A study by University of British Columbia's Avian Research Center showed that the larger eggs of hens fed a teaspoon of diatomaceous earth per cup of feed had larger yolks and more protein than those of hens fed the same diet without the supplement.