Finished measurements:
Keeping in mind that it is a different construction from the “normal” cardigan, here are some numbers -

Petite: Fronts (x2) 10” each, Back: 14.5” wide
Small: Fronts (x2) 11.5” each, Back: 14.5” wide
Medium: Fronts (x2) 13” each, Back: 16” wide
Large: Fronts (x2) 14.5” each, Back: 17.5” wide
Xlarge: Fronts (x2) 16” each, Back: 19” wide

The back panel is 11” for the P, S, M and L or 13.5” for the XL tall respectively lengthwise, and then you would add on the width of the front to complete the back, though, because of the way it drapes, the xl back is not really 32” long!