This paper considers the problem of determining standard product sizes. A customer, who does not find his size, may purchase rather larger or smaller one, but the purchasing desire decreases as the difference between the required and the prepared increases. Introducing a potential demand distribution and a loss function, which reflects how much the purchasing desire chantes according to the difference, we sormulate the problem and suggest a procedure to determine the optimal standard sizes minimizing the loss. Numerical examples are presented to explain the result.