Mine lay in theirs, but I have theirs on the floor so maybe that is the key. I also have a decoy egg that shows them where I want them to lay. I've noticed with people who have free range chickens that they usually find some place on the ground as well to lay in. I would suggest putting them as close to the ground as you can. As for size, mine are 12" x 12" and I've had no problem with it being too small, even for my cochin that I used to have.
Good luck!