How to fit a Mixtec frame
 

My wife is looking for a new bike. She is soon to be 66 and wants to go with a step thru frame. How are these sized and fitted? She is 5' 4" tall with an end seam of 30 inches.
TSAPP

Picture of something you've looked at to narrow down her search?

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.