By and Large

 By and large is originally a sailing term meaning "alternately close-hauled and not close-hauled." 

A ship that is sailing "close-hauled" is sailing as directly into the wind as possible (typically within about 45 degrees of the wind). The "by" part of the phrase means "close-hauled." (This "by" also appears in the term full and by,meaning "sailing with all sails full and close to the wind as possible.") 

"Large," by contrast, refers to a point of sail in which the wind is hitting the boat "abaft the beam," or behind the boat's widest point. A 1669 example of a variant spelling of "by and large" gives us a sense of the range implied: "Thus you see the ship handled in fair weather and foul, by and learge" (S. Sturmy, Mariners Magazine). 

The suggestion of a wide range carries over into the term's "in general" sense.