One medium egg can be substituted for one large egg. The minimum weight per dozen large eggs is 24 ounces, and the minimum weight per dozen medium eggs is 21 ounces.
Egg size makes a difference once two large eggs are needed. Two medium eggs can be substituted for two large eggs. However, if using small eggs, three small eggs need to be substituted for two large eggs. If a recipe calls for three large eggs, four medium or small eggs need to be substituted. For four large eggs, the cook should substitute five medium eggs or six small eggs.