The \emph{residual size} of a class of algebraic structures is the supremum of the cardinalities of the subdirectly irreducible members of the class. If there is no bound on the size of the subdirectly irreducible members, the residual size is said to be \emph{unbounded}. In this case the class is said to be \emph{residually large}, otherwise it is \emph{residually small}. If all subdirectly irreducible members are finite, the class is \emph{residually finite}.