Eggs are classified by size all over the world. The standard size of a chicken egg in the US is “large” and refers to an egg that is about 60 grams (from 57 to 64g, actually). Anything outside this range gets a different size designation: small, medium, extra large, jumbo. Since most commercially produced eggs fall into the large category, every recipe written in the US for US cooks assumes these as the standard size unless otherwise indicated in the recipe.
A large egg, cracked, yields approximately 3 1/2 tablespoons of liquid. The yolk will be about 1 1/2 tbsp and the whites will be about 2 tbsp. An extra large egg will have about 4 tbsp of liquid inside, while a medium egg will have only 3 tbsp. These differences have no effect whatsoever on omelettes, hard boiled eggs or many other savory dishes, but they can actually have a big impact on baked goods.
With a recipe that calls for only one egg, the difference between sizes may not be noticeable. Substituting 4 extra large eggs in a recipe that only calls for 4 large eggs will add an extra half-an-egg in volume to your recipe. This changes the ratios given in the recipe and can mean that you’ll end up with a drier or eggier finished product, one not quite as good as would have been achieved with the right sized eggs. Extra large eggs can be a good deal when it comes to eggs for breakfast, but if you know you’re going to be dealing with a lot of eggs for baking (especially in egg-heavy recipes), it is often best just to buy the size of egg dictated in the recipe itself (large, if not directly specified).
The size classifications vary a bit by country. In the UK, a large egg is slightly bigger than in the US. In Australia, it is slightly smaller. Keep in mind that these differences will not affect most recipes, but that when you are dealing with a large number of eggs it could become an issue and measuring or weighing your eggs is a good idea.