According to the Daily Mail, a size 16 woman's weight varies widely. The newspaper conducted a study examining four women who were all size 16 and found that the women ranged in weight from 148 pounds to 204 pounds.
This is in part because women who wear the same clothing size may have different frames, carry their weight in different proportions or be different heights. Additionally, women's clothing sizes vary significantly between brands, so a size 16 from one brand may fit very differently from a size 16 from another manufacturer. While a size 16 is average for women, there's no single weight range to link the size to.