Mary B. Kirn announces the opening of M. B. Kirn Real Estate Associates, a residential brokerage and property management firm. The new firm, located in Arlington, will have 10 sales agents on its staff . . . Builders in Reston signed 59 contracts between Nov. 8 and 14, making that the best sales week for the community in 10 years. Reston Land Corp. also noted that 553 new homes and condominiums have been sold there since January, exceeding the same period in '81 by 78 percent . . . Parents and other supporters of Straight, Inc., a drug rehabilitation program for young people, plan to raffle off a four-bedroom house in Gaithersburg to raise funds. The house, on Damascus Road, has been appraised at $193,500. Call 469-7175 for more information . . . The Evans Co. of McLean has gotten approval from the Arlington County Board for its planned Virginia Square Plaza, a medical/dental, commercial and retail complex on North Fairfax Drive. Evans plans to add two new buildings totaling 327,450 square feet to the site, which already contains two buildings with 80,000 square feet . . . Weaver Bros., Inc. will be the exclusive leasing agent for the Grey Drug Fair facility, a 71,000-square-foot facility on Edsall Road near I-395 in Fairfax. The building has been been eastern seaboard headquarters for Grey Drug Fair, which is moving most of its operation to the headquarters of its parent company, Sherwin-Williams Co., in Cleveland