Using your ruler measure out two sections that are 22 x 23 and make a mark using your pencil these two pieces will be designated as the top and the bottom of the nest box 2 using the same plywood board measure four sections that are 18 x 22 and mark them with your pencil. Numerous designs dictate a six or seven inch hole this is far too large and may cause barn owls to shun such a nest box other designs call for a hole as small as 4 1 2 inches although some barn owls can fit through barn owls vary in size with females being larger than males and american barn owls larger than those in europe. Below are the essential requirements for barn owl nest box plans what is a good nestbox design list of criteria for indoor barn owl boxes entrance hole optimum size 130 x 130mm minimum 100 x 100mm maximum 150 x 150mm floor area of nest chamber good size range 02 to 04 m 2 absolute minimum 016m 2 the floor of the box featured on this page measures 500 x 400mm giving a floor