Use USDA grade AA or A large eggs and weigh or measure the volume. I recommend pasteurized eggs in the shell, such as Safest Choice, especially for buttercreams.
The weight of the eggs and thickness of the shell can vary a great deal, even within a given weight class, as can the ratio of egg white to egg yolk. The correct amount of whole eggs, egg yolks, or egg whites is essential to the volume and texture of a cake. For this reason, it is advisable to weigh or measure the whole eggs, egg yolks, and egg whites.