Medium /large would be better than small ... Once they stay going broody and setting on eggs , they will be in nest ALMOST constantly and you DO want them comfortable . Imagine setting on a " SMALL TIGHT NEST " for 21 days . Plus other hens come behind them and lay in same nest at times and you DO NOT want your eggs gettign broken . Larger is always better , kinds like when you build a coop and run . BIGGER is naturally always better . DH is having to enlarge ours for the 3rd. time and PLUS build a searate coop and run for our bantams .
LOL thats why I like my plastic milk crates , large and roomy