1. A mask sizing device for determining sizing information for custom mask design of a facial mask, comprising: an image projecting unit for projecting an image onto a person's face wherein the image comprises at least a number of contact points of the facial mask, said contact points representing potential points of contact of the facial mask with the person's face; a control unit for controlling said image projection unit to adjust dimension, position and/or orientation of the image and/or to project an image including two or more sub-images having different dimensions, positions and/or orientations, a sub-image comprising at least a number of contact points of a facial mask; and a read-out unit for reading out sizing information comprising information about the dimension, position and/or orientation of the image and/or a selected sub-image.