NASA’s asteroid trackers estimate the giant space rock measures somewhere in the range of 1,443ft to 3,248ft (440m to 990m) across.
The calculations are based on 519 orbital observations since HL1’s initial discovery.
At the upper end of that estimate, the asteroid is about seven times larger than the Great Pyramid of Giza in Egypt.
More than 100 London double-decker buses could fit across the asteroid’s diameter.
If an object this size ever struck the planet, NASA claims the effects would have a “worldwide” impact.
NASA said: “We believe anything larger than one to two kilometres – one kilometre is a little more than one-half mile – could have worldwide effects.