Following a recipe's instructions regarding egg size could make the difference between a perfect cake and a cake that's a little too dry (or wet). Eggs add moisture to batters and mixtures, and when it comes right down to it, baking is pretty darn scientific. If you mess around with the flour/moisture ratio, your end product isn't going to turn out the way the recipe writer intended (and you'd hoped for).
A good rule of thumb: even though it seems like a small consideration, listen to what egg size the recipe calls for. An oversight here could amount to a difference in 2 or 3 tablespoons of moisture in your batter: enough to throw off a recipe. But let's get real for a moment: you also can't be buying cartons of various-sized eggs and, even more, many of your recipes won't even indicate an egg size. So what to do? Most U.S. recipes assume you'll use a large egg unless otherwise indicated. So this is often a safe bet. Stock up. Now let's get baking.