EVERY NOW and then, the American theater--which seems to get tinier with every passing season--is reminded of the virtues of thinking big. This year the reminder came from abroad in the shape of the Royal Shakespeare Company's epic, "The Life and Times of Nicholas Nickleby." And none too soon.

The trail-blazing two-part spectacle was the year's most significant theatrical event, and not just because it defied most Broadway statistics, fetched a record $100 per seat, and kept eager audiences enthralled for 8 1/2 hours with barely time out for a stretch or two. Its long-range importance came as much from what it implied about America's theatrical institutions--none of which has the financial support or artistic resources to attempt such a daring feat.

At a time when national penny-pinching and economic uncertainty have chipped away at the hard-earned gains of our regional and nonprofit theaters and transformed the commercial theater into a playground mainly for the well-heeled, here was a popular British troupe that could and did aspire to the stars. Two questions immediately leapt to mind. How do they do it? and Why can't we? No one had any hard answers. Indeed, most companies were worried about merely surviving, and none was very comfortable with the Reagan administration's assertion that private corporations would rush in to pick up the slack of a financially diminished National Endowment for the Arts. Still, rather like the contours of a distant island to a drowning swimmer, "Nicholas Nickleby" served as a beacon--mocking in a way, but inspiring in the long haul, assuming that the theater can navigate its way safely past the sharks of inflation.

Locally, the Folger Theatre Group seemed to be suffering the most. First, it abandoned its policy of presenting two new plays a year at the Terrace Theater, then saw its budget for Shakespeare whittled back to $1 million, and finally hired a new artistic director whose first official act was a tedious revival of a deservedly forgotten Restoration comedy, "The Rover." The National was holding strong with some big Broadway blockbusters ("Evita," "Children of a Lesser God") and some blockbusting prices, but behind closed doors, the mood was less assured. In September, the president of the National's board, Maurice Tobin, claimed that the theater's coffers were actually perilously low and threatened to terminate the management agreement with the powerful and prestigious Shubert Organization, which provides all the big-draw shows. The squabble was messy and protracted; by the end of the year, all that could be said was that an uneasy truce reigned.

Ford's Theatre was more successful than most in scaring up corporate money, but ironically found nothing really worthwhile to spend it on. As for the Kennedy Center, the one theatrical institution in town that can command national attention, it pursuaded CBS to pitch in $600,000 for a season of six plays in the Eisenhower, beginning with an austerely challenging production of "The Physicists." The alliance, while immensely welcome, underlined the prohibitive costs of mounting a play these days and the increasing importance of corporate producers over the independent angel, who is being squeezed out of the market. Conglomerates rarely foster great art, however. Solitary visionaries do. Hence a bind that can only get tighter in 1982.

One way out of the economic crunch, long understood and practiced by the Center's chairman, Roger Stevens, is to turn to stars with proven drawing power. Liz Taylor blazed the way in "The Little Foxes," earning in the process an estimated 15 percent of the box-office take, which Variety predicted would go as high as $11 million nationwide. Claudette Colbert also reappeared on local boards ("A Talent for Murder"), as did Yul Brynner ("The King and I"), Lilli Palmer ("Sarah in America") and Ralph Richardson ("Early Days"). The trend is bound to accelerate in the months to come. But the hitch with so many stars, Richardson excluded, is that the art of acting is usually less served than the cause of keyhole peeping. Most audiences turned out to see Taylor in the flesh, not Taylor as Regina Giddens, the role she was essaying with admittedly fitful results. A truly healthy theater puts its dramatic fare first, and prizes its actors to the degree that they serve to illuminate a given work.

The year's most consistently responsible theater, once again, could be seen at Arena Stage, which continues after 30 seasons to maintain a high level of integrity. From a commercial standpoint, its carousel of three new plays ("The Child," "Disability," "Cold Storage") was a disappointment, but there was something heady about Arena's willingness to risk time and reputation on thorny works, rooted in the complexities of contemporary life. With "The Suicide," Nikolai Erdman's black comedy about the common man in Russia, overlooked by the revolution and exploited by his friends, Arena brilliantly re-instated a play banned in its homeland for nearly 50 years. It was the best a local theater had to offer, followed closely by Arena's "A Lesson From Aloes," Athol Fugard's heartbreaking study of apartheid in South Africa.

Life was ever more perilous for the area's little theaters, which are financially marginal to begin with and find any drop in funding of seismic importance. Still, the New Playwrights' Theatre justified itself with Jim Leonard's chronicle of Southern Indiana life, "The Diviners"; the Studio had a sterling revival of "A Raisin in the Sun"; and the Round House Theatre came up with an unexpected Christmas enchantment in "The Butterfingers Angel."

On the musical front, "Evita" stood head and shoulders over the competition, which was not too difficult considering that "Oh, Brother!" "They're Playing Our Song," Brynner's 25th year revival of "The King and I," "Barnum" and "Annie" (for the third time) were the main challengers. Fortunately, Tim Grundmann continued to churn out his zany little musicals for New Playwrights', and Stephen Wade was still plunking his banjo in Arena's Old Vat Room after 50 weeks, which seems to be a long-run record for Washington.

Economics, however, dominated the thinking of both theater-doers and theatergoers alike. The former were forced necessarily to narrow their ambitions. The latter, with tickets prices sometimes running more than $30, simply became more selective in their choice of fare. TICKETplace, offering half price tickets the night of performance, came to the rescue in November. But welcome as it was for consumers, it pointed up a worrisome reality: More than ever, people are shopping on a show-by-show basis. That can only complicate the existence of our institutional theaters, which rely, both economically and psychologically, on those patrons who are willing, through subscription, to endorse a full season, not just the latest hit. A large subscription audience gives a theater the license to explore, to chance and to fail, if necessary. Without it, every show has to aim for the widest possible constituency. Safety comes first. The marginal play is shelved.

In such a climate, it is unlikely that a work as radical as "Nicholas Nickleby" would ever be attempted. Richly populated, adventuresomely staged and utterly profligate in its means, it was the child of reckless dreamers. The comparisons it invited were largely dismaying. If "Nicholas Nickleby" was a theatrical Everest, much of the year's theater in Washington (and America, for that matter) clung to far lesser slopes. And often by the fingernails.