Commercial eggs are sized according to weight, and there isn't such a great difference from one weight to another for a single egg. In quantity, however, and where the specific balance of liquids, fats and particular proteins makes a difference, such as in baked-goods recipes, the differences can lead to unwanted results.
Weight Versus Volume
The standards for egg sizes are set for the weight of a dozen eggs. Large eggs, which are called for in most recipes, weigh 24 ounces per dozen. Jumbos are 30 ounces and a dozen mediums weighs 21 ounces. The smallest eggs, peewees, weigh just half as much as jumbos. Out of their shells, it takes five large eggs, but also five mediums, to fill an 8-ounce measuring cup, but only four jumbos or X-large. A large egg without its shell weighs about 2 ounces and contains a total of 3 tablespoons of material.
Yolk Versus White
Two of the 3 tablespoons in a large egg is white and only one yolk. You can substitute the whites of two eggs for each whole egg called for in a recipe, but you'll have a little extra moisture and will be missing some fat, so baked goods may come out crisper and less tender. Adding a teaspoon of oil may help.
For a recipe that calls for three large eggs, you should be all right with three mediums; though in a cake, you might want to add an extra yolk for richness, or a dab of oil. If you have jumbo eggs, you need just two for this recipe. For four, five or six large eggs, substitute one more medium and one less large. The differences get a little larger when you're using just whites, such as for an angel-food cake, or just yolks. You'll need the whites of only five jumbo eggs to fill that 1-cup measure, compared to seven large eggs.
Use large eggs at least once so that you know the consistency of batter or dough and how it should taste when it's baked. After that, if you find yourself with a different size of eggs than usual, you can use whole or separated eggs to get about the same results. The more the recipe depends on precise proportions, however, the less success you may find with these adjustments. Cookies rarely call for more than one egg per batch, so a difference in size of the egg will be less crucial and more easily compensated for by adjusting such ingredients as flour. A rich butter cake, however, may have very delicate chemistry, and it may take you several tries to achieve satisfactory results, with slight differences in moisture and fat content.