Sizes are not specified in the pattern, but the following is suggested:

Birth to 2 months: size 2.75 mm/US 2/UK(Imperial) 12 needles, to make a 10.2 cm/ 4” bootee.
3 to 6 months: size 3.25 mm/US 3/UK(Imperial) 10 needles, to make a 11.4 cm/ 4.5” bootee.
6 to 9 months: size 3.75 mm/US 5/UK(Imperial) 9 needles, to make a 12.7 cm/ 5” bootee.

Any 3-ply or baby wool is suggested in the pattern.