This paper presents an approach to developing size ranges of products that have ergonomic requirements. Ergonomic requirements are a main source of semi-similarity in size ranges. This approach is based on a process model for uncertainty analysis, the size range development methodology of Pahl and Beitz, and robust design principles.The process model captures the interactions between user, the man-machine interface and the scaled product (the appliance). Interactions between user, man-machine interface and appliance can be classified as planned interactions or disturbances, and either influence or impact a variation. Using three robust design principles for elimination of disturbances and elimination or reduction of disturbance influence or impact, the synthesis of solutions for man-machine interface design can be supported. The robust design principles are adapted to the interactions of the user and the man-machine interface with the size range. The outcome is integration of the two disciplines, ergonomics and size range development.