What is the proper dosage of magnesium i should take for constipation? female, 21 years old, about 140 pounds.

Depends on type: There are many types of Magnesium. Magnesiums that are poorly absorbed, like Mag oxide, help by acting as a laxative. But magnesium deficiency can also cause constipation and well-absorbed forms like mag taurate and glycinate help. I advise trying 400 mg/day of well-absorbed forms, and you can increase dose if that does not help. High doses are safe. See http://tinyurl.com/m7oovm4 for details.