The Airfit N20 for her Headgear is only available in one size. The size that comes with the originally packaged mask is Small. All sizes of the N20 Standard headgear will fit all sizes of the N20 for her Mask.
It is important to determine if you have the Standard headgear of the Standard For Her version. The standard version has blue accents on the headgear and the For Her version has a light purple color. If you have a For Her version of the N10 Nasal Mask this Standard size is not what you are looking for. You will need to purchase the N10 For Her headgear specific to the For Her mask. You can find that product in the headgear section of the Mask Parts or in the related products.