E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To sail large is to sail on a large windi.e. with the wind not straight astern, but what sailors call abaft the beam.
Set at large, i.e. at liberty. It is a French phrase; prendre le large is to stand out at sea, or occupy the main ocean, so as to be free to move. Similarly, to be set at large is to be placed free in the wide world.