Even though some recipes call for large eggs you do not have to use a large egg. If all you have is medium eggs just use the same amount of medium that the recipe is requesting in large. The recipe will turn out the same. Only difference is if a recipe calls for 4 large eggs and I have medium I might use 5 if I have that many, If I have only 4 than I just use the four. I have seen no difference in the recipe either way.
One large egg is the equivalent of one medium egg. Two large eggs are the equivalent of two medium eggs. Three large eggs are the equivalent of four medium eggs and four large eggs are the equivalent of five medium eggs.