The O2 Nano Mask is currently available in two sizes: Medium and Large. The Medium measures about 9.7 inches across and 5.85 inches tall. The Large measures about 9.70 inches across and 6.65 inches tall. We typically recommend a Large for men and Medium for women, though this depends on head shape and preference. However, the sizing difference refers to the maximum length of the ear straps. Both sizes of the mask can be adjusted to fit a range of faces. Likewise, the nose bridge, adjustable straps, and hoops all help you to achieve a secure fit. We will also be announcing a Small/Kids size after the campaign.