Sizing of the frame is never too clear to me, unlike a diamond frame. Fitting is the same as for any other frame - saddle position, 'bar height per rider preference, et cetera.
Basically sizing for a mixte is based on the seat tube being long enough to hold the seatpost at the position she needs, the distance from the seat tube to the head tube long enough to set up the reach she needs, and head tube height appropriate to set up the 'bar/saddle relative heights where she needs them. A bad dimension might be too long or too short.