Patrick Herning has a vision for the future of plus-size shopping.

“Say you’re in Nordstrom with your straight-size friend,” he said, using the term for standard industry sizing. “She sees this Altuzarra blazer, and you can go to 11 Honoré for the same one: same brand, style, fabric, just a different size.”

That is his elevator pitch for 11 Honoré, the designer shopping site he founded in 2017 for sizes 12 and up. The company, based in Los Angeles, has helped labels like Diane von Furstenberg, Carolina Herrera and Rachel Comey venture for the first time into extended sizing.

Some luxury labels on the site, including Henning, a tailoring line, and Baacal, a sustainably made line designed by Cynthia Vincent, were introduced exclusively as plus-size lines. Others, like Dyvna, from Shirley Cook, the founding chief executive of Proenza Schouler, make clothes up to size 18.

And 11 Honoré isn’t averse to making a splash to get noticed. In February, it threw a pre-New York Fashion Week dinner party in the West Village attended by a gaggle of plus-size models and influencers. This followed a runway show in September made up entirely of models size 12 and above, led by the actress Laverne Cox in Zac Posen.

High fashion has, historically, not been a welcoming place for plus-size customers, even if Ashley Graham has made it to the cover of Vogue a few times. Culturally, women with bodies well outside the range of sample sizes were not embraced — Karl Lagerfeld, the Chanel designer, often made his ungenerous feelings about larger women known — and there were almost no places to buy well-made or fancy clothes, even if you had the money and the desire for them.

Why invest in clothes if your body was considered undesirable, or when you were one successful diet away from the necessity of wearing them in the first place?

“Plus-size women have never been told they could stay that way and invest in themselves as they are,” said Lauren Chan, who started Henning last year. “Before this chapter, I was a plus-size model and a fashion news director at Glamour. I had these incredible colleagues wearing Miu Miu every day like they had that armor on, but my weight fluctuated, and I wore from size 12 to 20. I was in meeting in C-suites in Condé Nast wearing Forever 21.”

Henning blazers have a trim on the lining with a pep talk that reads: “Wear it like you mean it.”

“SINCE I WAS A PRETEEN, I was a size 14 and up,” said Aidy Bryant, the “Shrill” star, “Saturday Night Live” player and a founder, with the costume designer Remy Pearce, of the plus-size line Pauline. “I feel like so much of my youth was going to the mall or stores with my friends and watching them shop and being like, ‘I’ll buy a hair clip.’”

Ms. Bryant remembered a photo shoot during her “Saturday Night Live” debut season, with Kate McKinnon and Cecily Strong. “They had these unbelievable dresses, and then I got to my rack and there were two things on it and they were both from Macy’s,” she said.

“I thought, something is not the same here. This isn’t a system I get to be a part of at all, and that’s hurtful. I can’t stand beside them and enjoy the same thing. The breadth of choice is just not there.”

Couture customers can have access to designer clothing for the right price, and celebrities like Beanie Feldstein may get to wear a Miu Miu gown to the Oscars. Still, plenty of famous women whose bodies are above a sample size, including Leslie Jones and Melissa McCarthy, have spoken publicly about the difficulty of finding designers to dress them for the red carpet.

There may be more plus options than ever — and some designers are nodding toward inclusivity by incorporating one or two plus-size models on the runway — but there is a very long way to go, as illustrated by the influencer Katie Sturino’s “Make My Size” series on Instagram.

In the series, Ms. Sturino, who wears a size 18, photographs herself in the dressing room struggling to fit into the largest size she can find from labels like Aritzia or Zimmerman or at Neiman Marcus in Hudson Yards, along with an open letter to the brand — and her nearly 500,000 followers — expressing her desire to spend money on their clothes.

“I get that if you’re starting a small collection, you may not be able to afford plus, but Zimmerman, Alice and Olivia, Tory Burch still don’t do it,” Ms. Sturino said.

Her campaign has had its share of success stories, as when she did a “Make My Size” post on the Veronica Beard line. “They look like fashion girls,” Ms. Sturino said, referring to the founding sisters-in-law, Veronica Swanson Beard and Veronica Miele Beard. “I didn’t think they would care or want to dress me, but they reached out and said it was something they were working on and thinking about. We went to lunch the next week.”

“Tanya Taylor is another one,” she said. Both lines have extended sizing.

The misstep that some designers make, Ms. Sturino said, is to offer only basics to their plus-size customers. “I don’t want another $19 pair of jeans,” she said. “A plus-size woman can buy black pants. The misstep that brands take is: ‘Oh these women don’t want to wear color.’”

MR. HERNING IS FOND OF SAYING that more than half of the United States population is the 11 Honoré audience — as of 2018, it was estimated that 68 percent of American women are size 14 or larger, according to Plunkett Research — but it is one that he, as a slim man, had not paid attention to until a few years ago.

He worked in fashion and event marketing with clients like Louis Vuitton and the Italian label Marina Rinaldi, which is owned by the Max Mara Fashion Group and has long been one of the few plus-size luxury lines.

“The idea for 11 Honoré was planted in 2016 when I worked on a project for Marina Rinaldi,” Mr. Herning said. “It is how I got to meet this customer, and I saw this underrepresented audience and an opportunity to dress a younger demographic.”

Clothing lines have long used the excuse that extending their size range is expensive, often citing the difficulty of developing new patterns. Anticipating those issues, 11 Honoré offers its own resources and expertise in pattern development or simply in helping a label to decide where to start. Diane von Furstenberg, for example, started with the signature wrap dress in three prints.

11 Honoré declined to divulge revenue figures. In November 2019, it secured $10 million in funding led by investors including Nordstrom and the venture capital firm Greycroft.

Dana Settle, a founding partner of Greycroft, has known Mr. Herning since he worked in marketing. “He came to me very early on with the concept,” she said. “I thought it was interesting and told him to do some homework and come back.”

When he came back to her, she realized, she said, that “this is a market that was really not understood, and completely underserved.”

Mr. Herning wants to open retail stores, though the company has showrooms in New York, Los Angeles, Atlanta, Boston, Chicago and Dubai where customers can shop in person. Later this year, it is introducing a private label.

“Great pants, pencil skirts, button up shirts, jeans,” Mr. Herning said. “I don’t like using the word ‘basic.’ It’s to complement investment pieces. The inspiration is the Row.”

Right now he has two wishes. One is for brands that offer limited sizing on 11 Honoré, including Ganni (up to size 14) and Dolce & Gabbana (up to an 18 that runs small), to expand to size 24. The other is for new brands to come into the fold: Gucci, Dries Van Noten, Stella McCartney.

Mr. Herning wouldn’t name brands he has approached that don’t want to venture into plus. “More than 70 percent have changed their minds,” he said. “I have yet to put a brand out there because so many have come back to me.”