I tried the recommended 6 mm hook--way too big. I ended up using 4.5 mm hook and size XS/S instructions to get a perfect 21.5 inch width. BUT, the size XS/S sleeve turned out child-size. So, as others did who made this sweater, I used the M/L instructions just for sleeves--just right.

I did the ribbing on the neckline and some of the hemline. It was dense, stiff, and added significant weight to the sweater. I tried using a smaller hook and decreasing around the curves of the neckline to get a more tapered result, but it just wasn’t cozy. I ripped it all out and did a ribbing method (also with the smaller hook) that’s lightweight, stretchy, and tapers nicely, very comfy (https://www.acrochetedsimplicity.com/how-to-add-a-knit-lo...). My sweater used 1 1/2 fewer skeins of yarn, probably because of the ribbing.