This is actually a legitimate anatomy question, so here's the straightforward answer.

The anal canal is quite elastic thanks to its muscular sphincters and surrounding tissue. In terms of practical limits:

Resting vs. maximum opening. At rest, the anus is essentially closed. The two sphincters (internal and external) can voluntarily and involuntarily relax, and the tissue can dilate considerably. During normal defecation, the diameter passing comfortably is typically around 2–3 cm. With gradual, deliberate stretching and lubrication, significantly larger diameters are possible without injury — which is why certain medical procedures work the way they do.

Medical context gives us good benchmarks. Anal exams routinely accommodate an adult finger (~2 cm), and colonoscopes are roughly 1.3 cm in diameter. In surgical and emergency settings, the anus can dilate to accept much larger objects — which is also why accidental foreign-body retention happens, something ER departments deal with regularly.

Practical and safety considerations:

• The internal sphincter is not under voluntary control; forcing things past resistance before it relaxes is what causes tears (fissures), which are painful and slow to heal
• Gradual, patient dilation with lubrication lets the muscles relax naturally
• Anything rigid or sharp poses real perforation risk to the rectum, which has thin walls and a curved anatomy
• Over-aggressive stretching can, in extreme cases, contribute to sphincter weakness or incontinence over time

The rectum itself is angled and connected to the sigmoid colon, so "how far" lengthwise is limited by that curve — objects pushed past it tend to lodge or cause damage, which is why ER retrieval is sometimes needed.

If you have a specific context in mind (medical, curiosity, or practical safety), I can go into more detail on that angle.

*From Lumo