Hello and Welcome To BYC! With mixed size birds in a coop, I usually just use 14" square boxes, the big birds are happier and the small ones don't really care. With silkies do Iike the first row of boxes on or not that far off the floor, with another row on top, do prefer a perch in front of that row for the girls to stand on, like a 4-6" lip on the bottom edge of the boxes.