wagtail/client/scss/components/forms/_form-width.scss

.w-form-width {
  @include max-form-width();
}

@include media-breakpoint-up(md) {
  .side-panel-open {
    .tab-content {
      width: theme('width.[2/3]');
    }
  }
}

.fields {
  // Apply the desired form width for legacy `fields` container.
  max-width: $max-form-width;

  // Remove any spacing in legacy fields markup.
  > li {
    padding: 0;
  }
}