Darling little Stephanie Mills, the 4-foot 10-inch singer who played the "Wizard of Oz's" Dorothy in Broadway's black musical version, "The Wiz," was mad as hell Saturday night.

First it was the lost costume: a red, gold, green and purple sequinted pantsuit that was replaced by a white sequined one-shoulder dress with a skirt of white and blue scarves.

She appeared from her dressing room at DAR Constitutional Hall tense, and marched up to the stage door yelling, "Come on, get it together. What side are they [her backup singers] supposted to be one? Let's go."

On stage her petite frame rocked, slinked and jumped in acrobatic feats that matched the vocal gymnastics of her strong, two-octave voice. She was funny, lovable and wickedly teasing, drawing some men down the aisles for kisses during her two concerts here this weekend.

But because the first act had run long, Stephanie's headline spot was trimmed to 40 minutes. She stayed on for 50 minutes and came off yelling again: "Audrey, they cut my songs."

"If you watch the show you'll see she's very hyper and energetic. She's that way just before and after, but really she's a very calm person," Audrey Mills, Stephanie's sister, had said earlier.

Calm. Quiet. A real homebody. Her mother and father, sister and brother, aunts and uncles agree.

Maybe Stephanie should have recorded "We Are Family." She is surrounded by family.

Backstage, her family, Joe Mills, used his burly frame to discourage would-be intruders of his daughter's dressing room.

"Ask her brother Joey," he said, referring all questions about the 23-year-old star who helped turn "The Wizard of Oz" into "The Wiz" on Broadway five years ago.

"See what Audrey says," replies Joey.

Audrey and Joey, Stephanie's older sister and brother, manage her on the road.

"This is a bad time because everything got so confused," Audrey says. "My mother lost part of Cookie's costume. She always comes on the road with us, but there's a system, you know, and she doesn't know it. So by trying to help, she sometimes messes us up."

When the fuss about the show's length is over, Stephanie appears cool and relaxed in slippers and yellow robe.

"I am a homebody," she admits. "I don't like going out much. I love good movies. And sports. I love sports, everything but baseball."

She's calm now, playfully hoisting a 4-year-old relative on her knee. Her brother Joey rolls her limp hair, replacing for the next show the curls lost in the first.

"She is spoiled," Audrey had also said earlier. "My sister and I did it.

When she was a baby we thought she was a doll and we treated her like one. We dressed her three times a day. But her being spoiled is the result of her being the baby. It's got nothing to do with her being a star."

Audrey is eight years older than Stephanie. Joey is 10 years older. There are another brother and sister older than Stephanie, then a younger brother. That makes Stephanie the baby girl.

The baby girl has been singing since she was 3. Her big brothers and sisters spent their time trotting her around from one talent show to the next in their home town of Brooklyn, then New York. She kept winning first place so they kept trotting her around.

At 9, she won the talent contest six weeks straight at Harlem's legendary Apollo Theater.

"The only way to get rid of her was to give her a week with the Isley Brothers," Joe Mills said.

A week's professional booking and she was a star, at least to her family with whom she now shares a comfortable home in Mount Vernon, N.Y.

The Apollo booking was followed a year later with her Broadway debut: a small part in "Maggie Flynn" with Shirley Jones and Jack Cassidy.

Then at 16 she became the Wiz Kid in the black Broadway musical. "When I Think of Home" was for her what "Somewhere Over the Rainbow" was for Judy Garland.

"I was hurt . . . but not jealous," Stephanie said of losing the movie role of Dorothy in "The Wiz" to Diana Ross. Ross was a commerical choice, decried by some critics and the public.

"But anyway, after I saw it I was glad I didn't do it. I didn't like it because it strayed too far from the original and from what I was doing on Broadway. I mean, I didn't want to see things like a garbage (dumps)."

Even if Stephanie had been jealous about the movie version, losing it to Diana Ross would have softened the disappointment. If Stephanie Mills idolized anyone it is Diana Ross.

"It's always people who are really accomplished or talented who don't realize what they've got is anything much," Audrey Mills said, explaining that her sister idolizes other singers but sees herself as just a little more than ordinary.

"Cookie doesn't look at what she's got as being special."

"No, I just see myself as somebody who likes to sing," Stephanie adds.

It is suggested that maybe she is as good as Diana Ross and that that night's rousing performance and crowd adulation are a testimony to that.

She looks astonished. "Really!"

Stephanie, who is on her second top 10 album since leaving Broadway in 1978, doesn't see herself back there for another three or four years. She likes the way things are now: no long tours, just a few concerts a week that she can travel to by bus with her familial entrourage.

"I like being on the road . . . weekedns. Like I'm doing now. Than I can go home when I'm done.

"Hey, I hear you're getting married," an aunt says, out of turn, Stephanie points out.

Aha, there is someting to go back to.

"That's not true," her mother, Christine Mills, cautions.

But there is confirmation in Stephanie's voice, a bit of rebellion in her sideways glance at her mother. Stephanie admits, "We were dating seriously."

She must be. She is preoccupied with the married life of a young woman she has met for the first time. She interrupts their conversation with questions:

"You're married? How is it? How long have you been married? Have you got any children? When are you going to start a family?"

"When are you?" she is asked.

"She's too young for that," her mother interrupts.

"Yeah. She's the baby girl all right," someone says.

Her mother stresses the point. "She is the baby."