Plus Size in Japan starts at the Japanese size 2L. For reference, a size 2L is about a US size Medium/Large, or 8/10. Women with a bra cup size larger than an American C are considered plus size. Other words for Plus Size in Japanese fashion are “Big Size” and “Queen Size”.
Someone who is tall may be considered plus size. Having bust or hip measurements larger than average can also classify someone as plus size.
The term plus size has been used in North America since the 1920s, but in Japan, the term プラスサイズ (written in katakana) is relatively new.
Japan has other popular terms for clothing that is larger than the Japanese industry-standard; Queen size, Big size (大きいサイズ), Chubby size. But these terms generally applied to the clothing only.
In English, Plus Size isn't just a way to label clothing. Over time it became attached to movements that promoted diversity in fashion and body acceptance. The idea of Body Positivity (ボディポジティブ) often comes to mind when discussing plus size in Japan.
Interestingly, in Japan, we're seeing models embrace the Plus Size label more so than the fashion industry. Unfortunately, when we're shopping in Japan, labeling and titles are still inconsistent for plus sizes.
When a foreigner comes to Japan, suddenly being labeled as plus-sized can be jarring if they're a relatively average size in their own country. Japanese clothing sizes are typically smaller than US sizes (see the international size charts here) and are tailored in a way that doesn't accommodate wide frames, long limbs, large busts or hips. In the 2022 survey of foreigners living in Japan, 66% of foreigners reported that they were unable to or had extreme difficulty finding clothing in Japan.