Wispy Cardi Sizing

A few knitters have asked for clarification on sizing for Wispy Cardi. The pattern includes this sizing information:

schematic

Sizes:

Child's 4/6 (8/10, 12/14, Women's S, M, L, XL, XXL)

Finished Measurements:

To fit chest/bust: 25 (28, 31, 34, 38, 42, 46, 50)"

Across back measurement (armhole to armhole): 17.5 (19.5, 22, 24, 26, 28, 30, 32)"

Sleeve circumference: 10 (11, 12, 13, 14.5, 16, 17.5, 19)"

How do you determine which size to knit based on the "across back measurement"?

To clarify, the measurements listed here are the actual measurement of the sweater, from it's armhole to armhole, NOT the measurement across your back from armpit to armpit.  To figure out which size to knit, put your arms at their sides.  (You might need some help with this one!) Start at the side of your left arm, then across the back of your arm, across your back, across the back of your right arm, finishing the measurement at the side of  your right arm.

Because the sweater is knit from side to side, the good news is you can try it on as you go and make adjustments.

I hope this is helpful.  Any other questions?