Thanos' penis size, based on fan calculations and proportional reasoning from his height, is estimated to be about 5.4 inches flaccid and possibly nearly 12 inches erect. This estimate is derived from scaling relative to actor Robert Downey Jr.'s height and known average human proportions, suggesting a flaccid length of about 5.44 inches which could double when erect to nearly a foot long.
For context, Thanos’ height varies depending on the source, but in the Marvel Cinematic Universe (MCU), he is about 8'3" (2.52 m) tall. These proportions have been used by fans to extrapolate the size of his anatomy. Another more conservative fan estimate, using a comic book height of 6'7" (2.01 m) for Thanos, suggests a penis length of about 6.3 inches based on average human ratio calculations.
Therefore, the popular fan consensus puts Thanos' flaccid penis size roughly between 5.4 and 6.3 inches, and erect size potentially much larger, approaching a foot in length according to some estimates.
