There is no maximum diameter, it just depends on how much you stretch it or insert an object to stretch it. I would say most people cannot take more than 3 inch diameter as that is a lot of stretch.
The anal canal has a ring of muscle, called the anal sphincter
The anal canal serves to transmit and lubricate stool as it passes externally from the rectum
The rectum, the final portion of the large intestine, becomes the anal canal.
The anal canal
Anoscopy is the visual examination of the anal canal and lower rectum.Anoscopyits actually proctoscopy
Assholicus Maximus & Buttockical Minimus
anal canal
The function of the anal canal is to transmit and lubricate stool as it bypasses externally from the rectum.
Approximately half of anal cancers are squamous cell carcinomas, which arise from the cells lining the anal margin and the anal canal.
Rectum.
anal canal
bulbospongious