Summer breeze shrug is knit in the round from center back out and is roughly elbow length as written. The pattern is written in sections in an abbreviated/outline type format

Sizes: XS (S, M, L, XL)
Finished Cross Back Measurements: 14” (15”, 16”, 17”, 18”)

To find your cross back measurement you measure from the outside of one shoulder to the outside of the opposite shoulder, across your back.

Measuring from armpit to armpit across your back will usually get you close enough and is a lot easier to do without help.