One of the things to consider when looking for costumes sizes such as “medium” have very little relevance. This is because different manufacturers have different opinionts on what size is. For example a size 10 – 12 can be considered both a “Small”, a “Large” or a “Standard”.
American costumes sizes can be very different from European and Asian ones – In america an Adult size can be a Size 2, in Ireland this same person may be a size 8 – 10, Most costume companies will do their very best to give you an accurate description of the “actual fitting size”.
Other costume designers will often make teen sizes all the way up to plus sizes of the same range. Rather than changing the packaging for teens, adults and plus sizes. So don’t get insulted if you recieve a costume with “Large” label on it, you might find the contents to still suit your needs.