WHEN A PLAY is small, you've got to do big things with it to make it live. Aleksei Arbuzov's The Promise has all the signs of a small work: brief scenes that allow little sustained action, a lean plot, and a theme that's nothing to write home about. It is more a character study than a play--the story of two teen-age boys and a girl who escape death during the tragic siege of Leningrad only to become failures as adults.

Still, for all its simplicity, a three-act visit with characters who see their hopes turn to ashes over a period of 17 years can provide emotional fire for the audience if all the parts of the production function properly. The happy news at the Loeb this week is that director George Hamlin has seen to it that everything about his Promise is right; a production that could have been a fiasco (as this play was in its Broadway version last year) is an often chilling piece of theatre instead.

From the moment the curtain rises, we know this fragile play is in good hands. The set, designed by Donald Soule, fills he theatre with the atmosphere of imminent despair; the room in which the players will see their dreams shatter is towering in size and dark in complexion. The lighting has the bleakness of doom.

But, at the Loeb, the audience has been conditioned to know that a good set doesn't necessarily mean a good evening. We have to wait and be sure that the actors don't screw everything up. For this production, Hamlin has chosen a cast of newcomers to the mainstage. I don't know where he found them all, but I doubt he could have done much better.

THE FOCAL POINT of The Promise is Lika, the immature 16-year old who discovers that two men love her and then marries the wrong one. Eleanor Lindsay, who plays the part, makes a stunning and no doubt difficult transition from a wide-eyed girl who recoils at the thought of sex to a sad housewife married to a bitter, unsuccessful poet. Miss Lindsay seems somewhat stiff and unsure of herself at times, but with a few more performances this should disappear.

Besides, she happens to be a beautiful little girl, an dthat makes me inclined to forgive any small mistakes.

Another fortunate casting choice is George Sheanshang as Leonidik, the poet. In the first act, he makes the perfect budding writer--soft voice, gay eyes, fluent hand motions. When, in the third act, the gaiety turns sour, Sheanshang becomes a horrible figure of pathetic impotency.

Counter to Leonidik is Marat, the impetuous dreamer who achieves his goals of becoming war hero and bridge builder, but forgets to find love along the way. In this role, Michael Sacks captures the character's lust for action in the first two acts, but let's down a little in the third. He doesn't quite get at Marat's feeling of emptiness in spite of success, and tends to substitute volume for incisive characterization.

Luckily, director Hamlin has compensated for the minor acting inadequacies and, particularly, for the script's lack of density. He has fulfilled more promises than Arbuzou's play ever made.