The length of many vibrators and dildos was sized between 4 and 6 in, and circumference was between 4 and 5 in. However, some companies featured products of a considerably larger size than others. CONCLUSION:
Results: The average insertive length of the compiled dildos (16.7 ± 1.6 cm) was 1 SD longer than natal functional erect penile length as reported in the literature (15.7 ± 2.6 cm); however, their average circumference (12.7 ± 0.8 cm) mirrored natal erect penile girth (12.3 ± 1.3). The average girth of vendors' top 3 largest-girth dildos ...
On average, mean circumference for bestselling dildos was 12.7 cm. This is slightly bigger than the average natural penis girth, which some research suggests is between 11.5 to 12.3 cm -- although ...
The human rectum is only about 5-7 inches in length, and there is a sharp turn between the rectum and the large intestine. If the dildo is soft, it is indeed possible to insert up to a surprising amount of the dildo in your ass. Damage can occur in the large intestine, just as it can in the rectum, but the likelihood of you noticing it is very low.
The same survey showed that climax-inducing capabilities drop off dramatically in anything bigger than 9 inches.
Apparently, the ideal size is from 7 to 8.25 inches in length and from 6.25 to 6.15 inches in circumference. Who knew?
My personal preference would be 9 to 10 inches and thick enough that I can’t quite close my hand around it.