Various terms exist.
Proctoscopy is a collective term for the examination of the anus, rectum and sigmoid colon (one or all of them).
Anoscopy is the investigation of the anus (only) using an anoscope.
Rectoscopy is the investigation of the rectum (only) using a rectoscope.
Sigmoidoscopy is an examination of the rectum and sigmoid colon using a ...
The size of biopsy samples of the mucosa from gastrointestinal tract, including the colon, is ~2 mm.
Biopsy depth of 1.5 mm was chosen from our studies of colon biopsies
on fresh surgical tissue and IRB studies, and because forceps cup
sizes from 1.8 to 2.4 mm were rarely associated with adverse events. (Gastrointestinal Endoscopy, 2017)
Anal Stenosis, I think, is commonly seen in newborns or infants.
This needs treatment by itself.
A adult person having "Anal Stenosis", if he/she is passing stools normally, then he can, as well go for Colonoscopy.
Colo-Rectal surgeons do surgeries, like complications Crohn's disease or Ulcerative Colitis or Megacolon, resection of Colon cancer etc.
If you are not pleased with what the doctor said or you do not trust him, obtain your medical documentation about your case and ask for the second opinion.
Another doctor, in order to be able to give you a proper advice, will need to know the exact type and size of the polyp, was it the only polyp, the first polyp, etc.
Here is an example of guidelines ...