Why is there no official record?
Guinness World Records does not maintain a public category for it, so there is no referee. What exists instead is a handful of widely reported claims, and the difference between a claim and a clinical measurement is the entire story. A clinical measurement means a professional, a documented method, bone-pressed, published. None of the famous numbers clears that bar.
What about the famous claims?
The best-known claim is 34.3 cm, self-measured and shown in documentaries but never verified by an independent clinician. Another widely covered case claims over 48 cm, but doctors who examined the man reported that most of that length is stretched foreskin and skin tissue rather than functional anatomy. In both cases, media coverage repeated the number and skipped the fine print. Our source-grading habit applies to record holders too, and here it hands out a D.
What is the largest clinically measured?
Large clinical datasets like Veale et al. 2015 put the erect mean at 13.12 cm with a standard deviation of 1.66 cm. Run that distribution and an erect 17 cm is already the 99th percentile, about one man in a hundred; 18 cm is roughly one in six hundred. Above 20 cm you are past one in fifty thousand, and documented clinical cases at that size are rare enough to be published as case reports. Verified numbers live a long way below the tabloid ones.
Does having more help?
Not according to the people involved. Partner surveys consistently rank size low among things that matter, and men at the far right of the distribution report practical problems the middle never thinks about: pain during sex for partners, positions that do not work, and condom sizes that ordinary shops do not stock. The far left and the far right of the curve have more in common than either expects: both mostly need decent information and a sense of humour.
What does this mean for you?
Nothing, statistically. Outliers are outliers; your comparison group is the middle of the curve, where nearly everyone lives. Drag the slider on the penis size calculator and see where you actually stand, expressed in men rather than headlines. Full citations are on the method and sources page.