This is the information I just gleamed for a CNN report on tv, so sorry no hyperlink right now:

Researchers from southwest university have developed a model for the formation of the Moon that correctly account for the current size of earth and its current spin rate. They have also identified the exact type of impact needed and Beleive that the size of the impactor was almost exactly that of Mars.

As well they have have derived the initial spin rate of earth before the impact.

Previous models predicted an object slightly larger than Mars colliding with earth, they also couldn't account correctly for earths' current spinrate, rather sugeesting it spin faster than it currently does.

-Justin Kelly