"distance between two objects," Old English span "distance between the thumb and little finger of an extended hand," probably related to Middle Dutch spannen "to join, fasten" (see span (n.2)).
The Germanic word was borrowed into Medieval Latin as spannus, hence Italian spanna, Old French espanne, French empan. As a measure of length, roughly nine inches. Meaning "length of time" first attested 1590s; that of "space between abutments of an arch, etc." is from 1725. Meaning "maximum lateral dimension of an aircraft" is first recorded 1909. Attention span is recorded from 1922.