TV

‘American Women’ actresses freaked out over their size 12 costumes

Alicia Silverstone and Mena Suvari were reportedly “horrified” when they were handed the clothes for their new show, the ’70s-era sitcom “American Woman.”

It wasn’t the outfits that appalled them, costume designer Judy Gellman tells The Post, but the size they were in: 12.

“Sizing was different then,” Gellman explains. Indeed, a size 12 then would translate roughly to a 6 today.

“I’d say, ‘You can’t look at the size, you have to look at the measurements.’ ”

Getting the clothing right is crucial to “American Woman,” airing Thursdays at 10 p.m. on Paramount Network.

Inspired by the childhood of its co-executive producer, Kyle Richards (“The Real Housewives of Beverly Hills”), the dramedy is set in 1975 Los Angeles. Silverstone stars as Bonnie, a trophy wife who leaves her cheating husband and is then forced to go to work to provide for her children. Naturally, she has to dress the part.

Gellman, who’s also worked on shows such as NBC’s “Bad Judge” and ABC’s “Super Fun Night,” was meticulous in researching just what it was Bonnie and her best friends Kathleen (Suvari) and Diana (Jennifer Bartels) would wear. She pulled more than 1,000 looks for the three ladies combined.

“As a costume designer, you want your work to be as accurate as possible,” Gellman says. “So I would say 98 percent of the clothes are real vintage.” She sourced the pieces through a mixture of estate sales, collectors and stores in LA and San Francisco. “Any vintage store that exists in Los Angeles, I’ve been in,” she says.

Gellman ended up covering all the walls in her office with 20 mood boards, plastered with photos of Los Angeles street styles from the ’60s and ’70s. And although she says she took most of her cues from back issues of Vogue and from the style icons of the time — Stevie Nicks among them — she also scoured Life magazines and even Sears catalogs to get a feel for “what the general population was wearing.”

One of her favorite finds was a pleated, robin’s-egg blue and pink Pierre Cardin dress from a San Francisco collector that Twiggy — the Gigi Hadid of her day — wore for a 1967 shoot for Vogue. Gellman is bummed that the dress never made it on-screen, but she’s holding out for a second season so that Silverstone can rock it, since its colorway sums up Bonnie’s vibe.

“The colors that were considered chic were very distinctive to that time period,” Gellman says. “With Bonnie, we used a lot of pastels — corals and mint.”

Gellman favors brands that were popular at the time, such as Diane Von Furstenberg, beloved for her bold prints and slinky wrap silhouette, and Holly Harp, who opened a hippie-chic shop on Sunset Boulevard in 1968 that was beloved by both Stevie Nicks and Janis Joplin. Gellman also sought out Cardin’s geometric silhouettes and futuristic patterns.

“American Woman” costume designer Judy Gellman shows off the production closet.Greg Gayne, Paramount Network

For Suvari’s character, Kathleen — a former Texas debutante trying to open a casting agency — Gellman opted for flirty prints, bright colors and form-fitting clothes to make her stand out. “She’s got sort of a cute, sexy way about her,” Gellman explains, “She really knows how to work the room.”

Bartels’ character, Diana, who works in a bank, has the most conservative look.

“Diana was a working girl who didn’t have anywhere near the money that either of those other two girls did, and she really wanted to be considered legitimate and qualified in a man’s world of a bank,” Gellman says. “Her color palette was more the basics: navy and beige and brown.” Her main brands? Anne Klein and Calvin Klein.

Just as important to the show’s fashion aesthetic is what Silverstone wears underneath her clothes. To keep things real, Gellman says she was determined to include lingerie by the late designer John Kloss, some of whose designs Silverstone wears on the show.

Silverstone — in one of her signature pastel frocks — in a scene from “American Woman.”

“He was pivotal in a sexier kind of look in lingerie,” says Gellman. There are no pointy bras or billowing panties to be found in “American Woman.” Instead, the era’s skin-baring scoop-necks and plunging wrap silhouettes required softer, less structured bras. Gellman says, “working women that were out on their own . . . were looking to be sexier in a more modern way,” which aptly describes Bonnie, Kathleen, and Diana.

Not only did the stars of “American Woman” contend with a different sizing system, but they also struggled with fabrics far less comfortable than cotton, silk and satin.

“We used a ton of polyester,” Gellman says. “It was very big then. When it came to things like bathing suits and jeans, they didn’t have a lot of give. The bathing suits that the girls wore didn’t have the same level of flexibility and stretchability that we have now.”

But as retro as some of show’s looks are, Gellman points out that the era’s aesthetic has never completely gone away.

“That yellow gold that Amal Clooney wore to the [royal] wedding,” she says. “That was very much a ’70s color.”