Arena Stage has appointed director Garland Wright to be the theater's first associate artist, as part of a major expansion of the company scheduled over the next few months.

Wright, 39, who guest-directed such Arena productions as "Happy End," "The Imaginary Invalid" and "The Tempest," will work for the theater 26 weeks a year and stage two offerings -- one of which may be Bertolt Brecht's "The Good Person of Szechwan" in October. He will also help plan the upcoming season and select new actors for the resident company, which will double in size from eight to 16 members.

Arena's producing director, Zelda Fichandler, described Wright yesterday as "an actors' director" and praised "his way of taking the inside of a moment and putting it on the outside so that everyone can see it." Wright directed the original production of "Vanities," which went on to become the longest-running nonmusical play in off-Broadway history. From 1980 to 1983 he served as associate artistic director of the Tyrone Guthrie Theatre in Minneapolis and his productions have been seen in most major regional theaters.

Wright joins Arena at a time when it is embarking on a significant period of growth, funded by a five-year grant from the National Endowment for the Arts to foster ensemble theaters. Under the terms of the sliding grant, awarded last September, Arena will receive $310,000 for the fiscal year running from July 1 to June 30, 1986, which it must match with $103,000. (In subsequent years, the sum provided by the NEA decreases, while Arena's financial participation increases. All told, the grant will generate more than $2 million, half of which will have come from the NEA.)

The grant allows Arena to offer 52 weeks of employment to its actors and artistic personnel (as opposed to the prevailing 40), hire artistic associates similar to Wright in the fields of play writing, design and music, and conduct workshops in the summer months, when the theater has previously been dark. The first three-week workshop, designed to develop and explore various scripts under consideration for next season, begins July 2.

Although his contract with Arena calls for a 26-week association per year, the weeks do not necessarily have to be consecutive, and Wright will be able to pursue outside work. The arrangement is likely to become the model for future associate artists at Arena. In the past, Fichandler has been obliged to hire guest directors and designers on a show-by-show basis. The NEA grant will be used specifically to forge the kind of continuing, but not restrictive, ties that she feels are most conducive to quality work.

This spring, Arena auditioned 1,200 actors in Washington and 11 cities across the country in its search for prospective members for the expanded company. The final selection will be made by the end of the month.