Dildred - from Carolina, Puerto Rico
It really depends on the recipe you are making, but in general you might not see a big difference substituting medium eggs for large (1 dozen large eggs is about 2-1/3 cups; 1 dozen medium eggs about 2 cups). If the eggs are used for leavening (e.g. angel food cakes), the cake might not rise quite as high with medium eggs. In cake batter, the texture might be a bit softer. Custards, puddings and cheesecakes might not set up as firmly with medium eggs as they would with large eggs.