NEW YORK -- Retail sales of magazines grew 5.4% last year to more than $3.9 billion, according to a recent survey of magazine wholesalers by the Council for Periodical Distributors Associations here. Supermarkets represented the largest retail category for the sales, capturing 41.5% of the market. In 1993, supermarkets had 43% of the market share. Convenience stores remained the second-largest retail category, representing more than 15% of the market, according to the survey. Their sales grew by more than 2%. Mass merchandisers or discount stores were the fastest growing category, with sales expanding by 28%. Mass merchandisers captured nearly 11% of the market share. Among the other classes of trade, drug stores had an 8% share; book stores, 6%; terminals, 4.9%; newsstands, 4.6%, and miscellaneous, 9.4%.