Sizing question? Here may be the answer!
The back width needs to be quite snug (lots of negative ease) so the wrap will, well, stay-put.
The measurements in the pattern are roughly the distance between the center point of both shoulder blades.
Though measurements need not be super exact, the following should prove to be a good guide:

BUST SIZE
32.5”-34.5”--- make 1st size
35”-37”--- 2nd size
37.5”- 39.5”--- 3rd size
40”-42.5”--- 4th size