Smallest cross-section of large bowel is seen at:
(Refer: Richard S. Snell’s Clinical Anatomy by Regions, 9th edition, pg no: 158)
Characteristic Features of Rectosigmoid Junction

The diameter of the large intestine narrows.
There is an absence of complete peritoneal investment.
There is no true mesentery.
The three taeniae coli diverge to form a continuous longitudinal muscle coat on the rectum.
There are no appendices epiploicae.
Endoscopically, an acute angle is encountered at the narrowing of the rectosigmoid and the rectal mucosa is smooth and flat, whereas the mucosa of the sigmoid forms prominent rugal folds.
Descending colon
Ascending colon
Transverse colon
Sigmoid colon