Normally, when a baking recipe says “eggs” it means large eggs. Large eggs are the standard of the baking world.
I don’t use large eggs. I use extra large. This is entirely determined by HEB only selling 18 counts of their good eggs in the extra large size.
For the most part this doesn’t really matter. I use this lovely and informative egg chart to determine how many eggs I ought to use.
Large is the standard, and normally extra large are used in a 1:1 ratio for large. But they are still slightly bigger, and a little more egg is a little more moisture and fat in a recipe. Recipes that call for whites will whip up bigger. Recipes that call for yolks will be more tender. I don’t know how much real difference it makes. One day I should do some comparisons.
Less useful, but still interesting is this other chart, which tells you how many whole eggs, whites, or yolks are necessary to make a cup for all the different sizes of eggs. Not many recipes list their egg requirements in cups, but occasionally you come across one that does.
Finally, one large egg white normally equals ~2 tablespoons and one large egg yolk normally equals ~1 tablespoon. There are always outliers here though. I’ve seen some mighty small yolks.
Experimental Cookery 