The present invention relates to a mask sizing device (10) for determining sizing information for custom mask design of a facial mask, wherein the mask sizing device (10) comprises an image projecting unit (20) for projecting an image (16) onto a person's face (18), wherein the image (16) 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 (18), a control unit (22) for controlling said image projection unit (20) to adjust dimension, position and/or orientation of the image (16) 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 (24) for reading out sizing information comprising information about the dimension, position and/or orientation of the image (16) and/or a selected sub-image.