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