from one of the deleted scenes from alien
here from
https://youtu.be/AvUbqY1S-QQaround 3:10
1200km
whether its 1200km to the planetoid circumference or diameter i dont know
BUT
thats the source i think. since it wasnt in the theatrical release i don't know if it counts for anything
oh sorry SM has the computer screen showing the quadrants with the distances and that is in the theatrical cut so i guess that s the official in movie canon source.