Then make both the run and the coop a bit bigger, or reduce your numbers to about 10 to 12, instead of 15. Think in terms of 'how much room can you give your flock', rather than 'how many chickens can I cramp in the space'. Nobody has ever regretted giving their flock too much space. The coop...
Is the run going to be covered so it stays dry? Will the coop be the only shelter? Do you live someplace where you get weather that will keep them inside the coop for several days at a time?