The 1840 edition accidentally, it would appear, dropped the repeated indefinite article a. Surprisingly, the 1858 Wright edition restored the repeated a (even though the 1840 edition served as the copytext for that edition). Perhaps the typesetter for the Wright edition had learned to expect the repeated a, especially given its occurrence elsewhere in the text (see the example under 1 Nephi 11:35 as well as the general discussion under conjunctive repetition in volume 3).
Summary: Maintain in Alma 46:3 the repeated a in the conjunctive phrase “a large and a strong man”, a distinctive characteristic of the Book of Mormon text.