The largest asteroid ever to hit Earth was probably Theia, a Mars-sized body that formed over 4 billion years ago, during the early days of the solar system. Theia formed in a Lagrange point, a gravitationally stable region in the Earth's orbit, located on the opposite side of the Sun. Eventually, orbital fluctuations caused this body to oscillate closer and closer to the Earth, and eventually, they collided, throwing up a greater volume of molten rock than all the present-day continents combined. Some of this molten rock was ejected so forcefully that it entered orbit, aggregated, cooled, and became our Moon. This scenario is called Giant Impact Theory, and it accurately explains most features of the Earth-Moon system, such as why the chemical composition of the Moon is similar to that of the Earth's crust.

Since Theia, the largest asteroids known to impact the Earth were much smaller, maxing out at around 10 km (6 miles) in size. Larger asteroids may have hit the Earth during the period called the Late Heavy Bombardment, which occurred between 4.1 and 3.8 billion years ago. Because most of the crust from this period has either been subducted back into the mantle or covered in layers of sediment and volcanic rock, the largest asteroid craters may be hidden.

The largest verified crater on the Earth today is the Vredefort crater in South Africa, also known as the Vredefort Dome or the Vredefort impact structure. The crater is 300 km (186 mi) across, about twice the size of the Chicxulub crater, left behind by the asteroid that exterminated the dinosaurs. The Vredefort crater is thought to have been made by the largest asteroid to impact the Earth besides the one that created the Moon — strictly speaking, Theia was a planetoid rather than an asteroid — about 10 km (6 miles) in size. Realistically, these are the largest asteroids likely to impact the Earth. So, when you see mentions of asteroids "the size of Texas" in movies like Deep Impact, realize they shouldn't be taken too seriously.

The Wilkes Land anomaly, a concentration of mass under the Wilkes Land ice cap in Antarctica, may indicate an even larger impact structure, but at the time of this writing, its nature is uncertain.