I think the reason for the lack of responses here is the fact that there's no "hard and fast" rule for the size dictating whether it's an Adult or Youth Mask. The size alone does not always mean it's strictly for kids or adults.
I think a better indicator is the quality and construction of the mask. If the bent wire is heavy gauge, it's more likely an adult mask. The kids masks used thinner gauge metal. Same for the padding... cloth padding or a sparse amount of leather padding most likely indicates a child's mask. Whereas thicker/more abundant padding is usually indicative of an adult model.
Yes, the size matters, but there were many different styles and sizes throughout the years. Earlier masks (pre-1900) tended to be smaller in size than the giant masks of the 1930s - 40s. Some adult masks were more elongated, and had throat/neck protectors... which would very typically indicate an adult model. Some Masks were extra wide, to offer an extended field of vision. Some had ear protection that extended the width of the mask by 2" or more (as opposed to masks without the ear flaps).
So whether it's an adult or youth vintage mask, there can be a significant amount of variance in the size, based on the mask's style and the type of protection it offered. Thus, there is significant crossover between adult and youth models.
If you did a Venn Diagram (with one circle representing adult masks and another circle representing youth masks), you'd see a shared area that represents at least 25% of the total space.
But if you still want to attach specific sizes to adult vs. Youth models, I will say that if the mask is over 9" tall by 7.5" wide, it's probably an Adult Mask. Just keep in mind that there are exceptions, and that earlier adult masks can be smaller than that measurement, while being perfectly legit. If you're still with me, I hope this helps!
