This varies from brand to brand and by country. Some stores will consider a medium to be a size 10, while others a size 12-14. As an average, a Medium is likely to be a around a size 12. Generally a Small will be an AU 8-10 and a Large an AU 14-16.
Sizes range from 0-2 (extra small) to 14-16 (extra large). Smaller sizes are usually petites, and larger sizes usually found in women's or plus size departments. Remember that each clothing brand will have their own sizing guidelines, so watch carefully to ensure you are buying the correct size.
While a size XL is designed to fit a size 16/18, she may prefer to wear a size 1X which equates to a size 14/16. This is because the plus size section was designed with curves in mind.
As an average, a Medium is likely to be a around a size 12. Generally a Small will be an AU 8-10 and a Large an AU 14-16. An Extra Small is likely to be a size 6, with an Extra Large likely to be a size 18.
US size 12–14 is usually considered “Large” and some brands do start “plus sizes” at 12. However, “plus sizes” usually begin at 14 or 16 (Which I believe is a misnomer; in reality, 12–16 sizes are actually pretty average.)
Plus-size clothing is generally considered a women's size 18 and beyond. “Missy” or “straight” sizes range from 00–16/18, XXS–XXL whereas plus sizes range from 12W–28W, 0X–4X and greater. You might think plus sizes are just extended straight sizes, but guess again.
US 14 - approximately UK 16 (Waist 33"/34" and Bust 41"/42") US 16 - approximately UK 18 (Waist 35”/36" and Bust 43”/44") US 18 - approximately UK 20 (Waist 38”/39” and Bust 46”/47”)