A dimension-specifying device specifies dimensions of an attachment of work equipment including an arm and the attachment. In the work equipment a first connection portion or a second connection portion provided at the attachment is connected to the arm. The dimension-specifying device includes a dimension storage unit and a dimension calculation unit. The dimension storage unit stores first dimensions of an attachment when a first connection portion is connected to an arm. The dimension calculation unit calculates second dimensions of the attachment when a second connection portion is connected to the arm on the basis of the first dimensions.