Where did the term "at‑large" originate from?

Large is a word with considerable history and many meanings, including "Having few or no restrictions or limitations; allowing considerable freedom. Also said of persons with respect to their thought or action" (OED, sense I, 11a). At large (with no hyphen) is an idiom stemming from this, meaning at liberty. It is possible to say at more large, meaning 'at greater liberty' or to use the verb enlarge to mean 'set free'; but these two are rare and not to be used without care.

"At large" is not hyphenated in this context (the suspect is at large). But there are phrases such as "ambassador-at-large" where the entire phrase is hyphenated. This means something like "a roaming ambassador", i.e. one not attached to a single country, but who plays a wider global advisory role on an issue. ("Ambassador-at-large for human rights", etc.)

Large is an old sailing term - (actually, we still use it on tall ships. When the wind is behind your boat (abaft the beam), it is called “sailing large.” In this favourable 'large' direction the square sails are set and the ship is able to travel in whatever downwind direction the captain wants, so “at large” would mean free to run with few obstructions - like a criminal at large. (The opposite of "sailing large" is calling sailing “by the wind” or “by the bowline,” and involves using the non-square fore and aft sails to tack into the wind. The sheets (lines attached to the control corners of these sails) are tied with bowlines ( bowline is pronounced bolin) . So, a trip planned to include both upwind and downwind sailing would “by and large” get you to your destination.