Here the 1858 Wright edition accidentally dropped the repeated indefinite article a in this sentence. The 1908 RLDS edition restored the a to the RLDS text. There are several other places in the text where a repeated a has been omitted from conjunctive phrases involving the adjective large; for a list, see under Mosiah 27:7. The critical text will maintain the repeated a here in Helaman 1:15. Also see the general discussion under conjunctive repetition in volume 3.
Summary: Accept the repeated a in Helaman 1:15 (“he was a large and a mighty man”), the reading of both manuscripts and the early editions.