Size is not mentioned in the pattern but the bootees can be made in 3 sizes by altering the size of the knitting needles.
Birth to 2 months: use one pair 2.75mm needles
3 to 6 months: use one pair 3mm needles
6 to 9 months: use one pair 3.25mm needles

These sizes will produce bootee/foot lengths of 10, 11 and 12 cm respectively.

1 25g ball of Shepherd 3 ply machine washable wool will make 1 pair of bootees (with wool to spare).