Arena Stage has more than doubled the size of its company by hiring 11 actors -- among them four local performers -- as part of a major expansion program, partially financed by the National Endowment for the Arts.

The new actors join seven veteran company members to give Arena its largest pool of resident talent since the late 1960s. All 18 will be employed 52 weeks a year; up to now, 40-week contracts have been the rule.

"These actors will help infuse the company with creative volatility and energy," Douglas C. Wager, Arena's associate producing director, said yesterday. "With them, we're really bolstering the younger end of the company in a significant way. These are 11 people who are excited about the notion of an acting ensemble and want to make a long-term commitment to Arena."

Many of the new company members have already appeared as guest artists in Arena productions, but some are recent graduates of various acting schools. Their hiring -- at annual salaries ranging from $18,000 to $25,000 -- concludes an extensive search conducted this spring by Arena personnel in Washington and 11 cities across the country.

The four local performers joining Arena are: Ralph Cosham, formerly a member of the Folger Theatre Group and a familiar presence in Washington since the days of the Washington Theatre Club; Cary Anne Spear, who has acted with such groups as the New Playwrights' Theatre and Horizons; Kim Staunton, a graduate of the Duke Ellington School of the Arts, who made her local debut in Arena's recent "Execution of Justice"; and Maggie Winn-Jones, a seven-year veteran of the St. Louis Repertory Theatre before coming to the Washington area.

Other new full-time members are: Casey Biggs, who played Dan White in "Execution of Justice"; Randy Danson, previously seen at Arena in "The Three Sisters" and "The Child"; Tom Hewitt, a member of the company for the 1983-84 season, who subsequently left to costar with Louis Jourdan in a national tour of the musical "Gigi"; and Tana Hicken, a frequent performer at Baltimore's Center Stage, who also appeared in Arena's productions of "Execution" and "Passion Play."

The company members fresh out of school are: Heather Ehlers, who trained at New York University's Tisch School of the Arts; and John Gegenhuber and Thomas Quinn, both graduates of the DePaul/Goodman School of Drama in Chicago.

This week, the expanded company began a series of summer workshops designed to refine material that may eventually find its way onto one of Arena's stages. One project will explore the theatrical potential of the Sinclair Lewis novel "Main Street." Wager also plans work on John Guare's "Women and Water," the newest script in the playwright's four-play Civil War series that already includes "Lydie Breeze" and "Gardenia." "Women and Water" will be staged during Arena's 35th season, which officially opens in October with Bertolt Brecht's "The Good Person of Szechuan."

Although the Arena company is larger than it has been in years, Wager said about half of the roles in the upcoming eight-play season will still be cast with outside actors. "Even with 18 people, when you're running two theaters, you have to draw on the individual talent pool," he said.

The current growth at Arena has been facilitated by a $310,000 matching grant from the NEA, specifically aimed at fostering acting ensembles. Longtime Arena regulars -- Stanley Anderson, Richard Bauer, Terrence Currier, Mark Hammer, John Leonard, Henry Strozier and Halo Wines -- have all signed for another season.