well that’s hard to answer … and more experienced members should answer it, but as I know the anal hole basically can not stretch any wider than Pelvic inlet
https://en.wikipedia.org/wiki/Pelvic_inlet
that’s quite large and is larger in women, besides the pelvic bone itself is made of two separate parts joining together under the pubic area with Pubic symphysis which is a joint that In normal adults it can be moved roughly more than 2 mm and with 1 degree rotation with unpleasant pain. This increases for women at the time of childbirth.
https://en.wikipedia.org/wiki/Pubic_symphysis
and about how deep it can goes, after passing Rectum (is about 12 centimetres (4.7 in) long) https://en.wikipedia.org/wiki/Rectum
you will reach to Sigmoid colon which is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus sigma + -oid).
This part of the colon normally lies within the pelvis, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity.
that’s why when you passing rectum and going upstairs you should turn left
you can see Roxy’s sigmoid colon clearly in this video
https://www.pornhub.com/view_video.php?viewkey=ph589e16674203f
here you can find average Diameters of the large intestine
https://en.wikipedia.org/wiki/File:Diameters_of_the_large_intestine.svg