In underwear:
5-S
6-M
7-L
8-XL
9-XXL or 1X in plus size
10-2X
11-3X
In Lane Brayant sizing, their 14/16 plus size is equivalent to a size 9 panty.
If their 18/20's are too big, the 14/16's will fit you JUST FINE.

My favorites for a long, long time were the Lane Bryant cotton hipsters and cotton thongs!