The largest objects we ever landed on a body was the Apollo lunar lander.
Not quite. Several Saturn V third stages had crashed on the Moon post-mission, not to mention the Centaur upper stage from the LCROSS mission, which were significantly larger than the landers. Which was why I upped my estimate of the size of a building on Earth that would house all the artificial objects on extraterrestrial surfaces. And I'm probably being extremely generous in my size estimate as well, as you've rightly mentioned.