Living conditions, temperatures, feed and hen weight determine the size of the eggs that are produced. For instance, a hen that lives in a hot climate lays smaller eggs than one who lives in a cooler environment. Size is determined not by an egg's circumference, but by its weight. While large eggs average 24 ounces per dozen, medium eggs only average 21 ounces per dozen. Although the difference in weight is not considerable, from a baking standpoint, every ounce matters.

For recipes that require one, two or three large eggs, substituting medium eggs will do. In recipes that call for four large eggs or more, substitute a matching amount of medium eggs, plus one. For instance, five medium eggs are necessary for recipes that require four large eggs. If substituting medium eggs for large in a recipe that requires five large eggs, add six medium eggs. For six large eggs, substitute seven medium eggs.

Add five medium eggs to recipes that call for a cup of whole eggs -- the equivalent of five large eggs. Baking recipes that call for a cup of egg whites, require the whites of seven large eggs. If you only have medium, add the whites of eight eggs. To substitute medium eggs for large in recipes that recommend a cup of egg yolks, add the yolks of 16 medium eggs -- two more than the required 14 large eggs.

A rule in baking that is commonly overlooked is the use of room-temperature eggs. Regardless of size, eggs straight from the refrigerator may cause the batter to turn lumpy, adversely affecting the texture of whatever you are baking. Set cold eggs in a bowl of warm water for 10 to 15 minutes to bring them to room temperature, or remove the eggs from the refrigerator 30 to 60 minutes prior to the start of baking.