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 include two or more sub-images in the image, each sub-image having different dimensions, positions and/or orientations; 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, and wherein the image includes an outline of a cushion of the facial mask or an outline of forehead pad of the facial mask.