The GS Style Guide: 7. Sizes

HOW TO CHOOSE THE RIGHT SIZE OF HOSIERY

There's a vast array of tights, hold-ups and stockings today, but the massive choice doesn't always mean ending up buying a correctly fitting pair. Most tights and stockings are sold in sizes small, medium, large and extra large and are based on height, dress size and hip size. 'One size fits all' tights only usually fits
M/L wearers.

Selecting hosiery that fits properly is one of the best ways to ensure maximum wear life. If the hosiery is too tight, it is more susceptible to wear and tear, and it minimizes flexibility and comfort. Sizing differs from brand to brand. That is because many brands use different yarns from different sources with different characteristics, and also may use different machinery and knit fabric constructions. So always check the size grid or chart of each product.

For the upper end of a size range, it's often wise to get the next size up for maximum fit and comfort. With shiny tights it is better to wear the correct size in order to stretch the lycra to create a high shine.
Properly fitting hosiery should not bag at the ankles or droop below the gusset. Full-figured wearers or those who are plus size or tall should look for larger-sized products for these different requirements and demands from the hosiery for a good fit rather than for petite or regular sized wearers.

Keep in mind the following:
The sizing charts used by various different manufacturers are likely to vary. There is no unified international sizing system, so it might be one size in one brand but another in different one. Size charts are only there to give guidance on how to find the best size. At the end what matters is the fit of the hosiery and how it makes the wearer feel.