CHICAGO — Jovana Savic kept leaving vintage markets empty-handed. At a size 18, she couldn’t find clothing that fit. Extended sizes were easy to find on eBay and Depop, but walking into a store and trying on a pair of jeans felt like an impossible task.

Even major retailers that sold plus sizes online didn’t carry those options in their stores. Ms. Savic thought, “How special would it be for fat people, tall people, people who are midsized, who generally feel like they have to go to the back of the rack to see if something’s maybe in your size — how empowering could it be to walk into a space where it’s all for you?”