Hello CedarSmoke. In baking, egg size matters because eggs provide moisture, structure, and fat. Standard baking recipes in the US and many other regions are almost universally developed using large eggs.
Here is a simple conversion guide based on standard egg volumes:
For 1 large egg, you can safely use 1 medium egg or 1 small egg without noticing a difference, especially in a quick weeknight dessert.
For 2 large eggs, you can use 2 medium eggs or 3 small eggs.
For 3 large eggs, use 4 medium eggs or 4 small eggs.
For 4 large eggs, use 5 medium eggs or 5 small eggs.
If you want exact precision, a standard large egg without the shell weighs approximately 50 grams (or roughly 3.25 tablespoons). A medium egg without the shell weighs about 44 grams, and a small egg weighs about 38 grams. If your recipe calls for multiple large eggs and you want the exact amount of liquid, you can simply crack and whisk your medium or small eggs together in a bowl, then measure out 50 grams (or 3 tablespoons plus 1 teaspoon) per large egg required.
For most quick weeknight bakes like cookies, brownies, or simple snack cakes, a 1-to-1 swap with medium eggs works completely fine if the recipe only calls for one or two eggs. I hope this helps with your dessert tonight!