The solar sail is required to only weigh 50 kg for 3125 square meters.
But a recent advance involving carbon nanotubes can produce thin sheets of arbitrary size for which a 1 kilometer square sail, or 1000m x 1000m = 1,000,000 square meters, would only weigh 30 kg:

Researchers produce strong, transparent carbon nanotube sheets.
"Strength normalized to weight is important for many applications, especially in space and aerospace, and this property of the nanotube sheets already exceeds that of the strongest steel sheets and the Mylar and Kapton sheets used for ultralight air vehicles and proposed for solar sails for space applications, according to the researchers. The nanotube sheets can be made so thin that a square kilometer of solar sail would weigh only 30 kilograms. While sheets normally have much lower strength than fibers or yarns, the strength of the nanotube sheets in the nanotube alignment direction already approaches the highest reported values for polymer-free nanotube yarns."
http://www.physorg.com/news5890.html

The sheets do not have the strength of individual carbon nanotubes but are stronger than steel on a per weight basis.

Bob Clark