This pattern is a modular design (join as you go). As there are no buttonholes needed, and you will be using the stitches as button holes, I recommend 1” buttons. Buttons are optional though, the pattern is the same either way. Here is the yarn recommendations for each size in yards:

XS=950 yards
S=1000 yards
M=1050 yards
L=1100 yards
XL=1150 yards