On record the smallest waist was..

… 33 cm (12.99″) set by Ethel Granger of England in the early 1920’s due to her husband’s obsession with corsets and piercings.  She also pierced her nipples, nose, and had huge piercings in her ears.

(Ethel Granger’s 1920’s 13″ waist when she set the record.. and Ethel about 10 years later)