One size fits most men and women.
Headband is 2" wide in front and tapers back for good ear clearance
Halo II pullover fits comfortably head sizes with circumference between 53 cm (21") (0.2 kg tension pressure) and 60 cm (23 1/2") (1 kg tension pressure)
Headband needs to be snug enough to form a seal against your skin, so it might work less efficiently on smaller heads. On the other side, it might be too tight for large heads. If you have circumference of the head more than 60 cm (23 1/2") or don't like the tight fit, you may consider Halo I Tie version, Halo V with Velcro or Halo X3 pullover in size L/XL.