Etymonline says:

c.1200, "bountiful, inclined to give or spend freely," also, of areas, "great in expanse," from O.Fr. large "broad, wide; generous, bounteous," ... An older sense of "liberated, free from restraining influence" is preserved in at large (late 14c.)

Wiktionary references the Old French au large ("at liberty", now means off in the sense of offshore/off the coast or abroad) in the Etymology section of its at large entry. I checked Google books, and in one commentary on Paradise Lost, at large is discussed:

The old meaning of large (liberated) may be found, then, to be preserved in at large as well as in the French.