ACTOR Fred Gwynne is a biiig man -- 6 feet, 5 3/4 inches, to be exact--and still growing professionally.

At 56, he has an equally imposing record of Broadway and television roles, including Officer Francis Muldoon on the 1961 comedy series "Car 54, Where Are You?" and as the gentle giant Herman Munster in the sitcom "The Munsters." He is currently starring in Anthony ("Sleuth") Shaffer's new comedy/mystery "Whodunnit?" The play has been roundly booed by the critics, but Gwynne has been warmly received.

Despite the less-than-glowing reviews "Whodunnit?" is receiving (one review ran with the headline "Whydunnit?"), Gwynne remains ebullient and loyal about the new show, which he calls "absurd, scary fun."

"It's something I've always wanted to play--the inspector from Scotland Yard, one of the things all actors want to do," Gwynne says. It's his day off from the show, and his booming but refined basso voice sounds a bit weary. "It's an archetypal thing, one that I've never done. Like, 'Big Daddy' in 'Cat On a Hot Tin Roof' is archetypal, 'Our Town' . . . Herman Munster, for God's sake, he's archetypal.

"Occasionally I B.S. about quitting the business and going back to what I started with," muses Gwynne, who is fond of inventing aphorisms in mid-conversation, such as "I think acting is trying to make believe you like adversity" and "The older you get, the more clearly you remember what it was you wanted in the beginning."

What Gwynne wanted in the beginning, he says, was to be an artist. He was a painter and sculptor before his acting career began at age 23. "Of course, at this age, I can't do it as a hobbyist," Gwynne says. "I'm not gonna amuse myself keeping the door open in the art world till I'm ready to stop acting. I don't have the time or the energy. I have to save everything for the show."

After a stint in the Navy, Gwynne tried his talent and patience at a New York art school. "After a year of it, I decided I did not want to be posthumous, and I didn't want to do commercial stuff, so I switched my major to acting."

His first Broadway role was opposite Helen Hayes in "Mrs. McThing." He's gone on to do "about a dozen" Broadway shows, including the "archetypal" Big Daddy, Col. Kincaid in Preston Jones' "Texas Trilogy," the manager of Macy's in "Here's Love," a musical version of "Miracle on 34th St." And he's been a veritable staple at the Kennedy Center. "I'd say I've been there more than any other actor working in American theater," Gwynne commented.

Gwynne began doing live television in 1951 and appeared in "The Big Heist," which was the first taped show out of New York City. He has consistently appeared on TV since then, doing guest shots and several PBS productions, including "The Police."

His two hits, "The Munsters" and "Car 54," were a mixed blessing, Gwynne says now. "It was a lot like a line from Noel Coward, I wish I could quote him perfectly, 'You just learn the lines as best you can, say them briskly and go home.' Theater is a job."

After two years of playing father to the frightening family Munster, Gwynne stuck it out in Hollywood for a year, "just to see if I was going to be 'typed.' When I was only offered stuff like 'Lost in Space,' I hustled right back to the East Coast. Being typed in Hollywood made it a necessity to come back and get my ass in gear on the stage and get over all that. I was ready to hold a spear."

Gwynne's height has been both a help and a hindrance, he says. "Sure, there were times when I didn't get roles because I was too tall. I went up to meet the producer for a show called 'Frogs of Spring.' I met him on the way up the stairs to his office, and he said, 'Oh, no, please don't waste my time. You're just too tall for the part. You saw the girl going downstairs? You would be playing next to her.'

"Well, my friend insisted that he had to at least hear me read, and after I read, the director, Burgess Meredith, came onstage and kissed me," Gwynne says with a laugh. "So it just shows that when you're an actor you can't let anything get in your way.

"But everything comes around," Gwynne says. "Now, I think everyone is taller. I think I've finally waited them out. All the milk and vitamins have finally paid off."