The mannequin needs to be considerably smaller than the interior of the costume so that it can be padded up to conform to the body shape of the original wearer. The best book we have found for this is http://books.google.co.uk/books/about/A_Practical_Guide_to_Costume_Mounting.html?id=GPgpRCvT_dkC&redir_esc=y
which is out of print (but available online) and a small bustform. In the UK we use Proportion London Workroom bust forms such as http://www.proportionlondon.com/bg2-6-43-Workroom-children and pad them up with polyester wadding covered with unbleached cotton jersey and soft cotton calico. You can also get ethafoam mannequins and carve them down to size. Vital that the mannequin materials are chemically stable and inert and don’t have hard edges which could chafe and damage fabrics.
Hope this helps.