What is the substitution extra large eggs in place of large ones. Recipe for lemon curd calls for large eggs. I only have extra large eggs

In a recipe like Lemon Curd, I just sub extra large eggs for the large. If I were making a very large batch, I would probably just leave out an egg or two.
In a cake, you probably should go by weight. All my recipes are formatted for el eggs, as that's what I usually have.

As Susan W said the difference is weight of an extra large egg to a large egg is less than a quarter ounce. That said, the yolk only takes up about 1/3 of the total weight of the egg. The difference between the yolk of an extra large egg vs a large egg is only 1/12 of an ounce. Unless your recipe calls for more than a dozen eggs the difference is completely insignificant.

A large egg weighs 2-2.25 oz. A jumbo or extra large weighs 2.5 or greater. The difference isn't huge, but you could remove some of one of your eggs.