discourse/app/assets/stylesheets/common/base/composer-user-selector.scss

div.ac-wrap.composer-user-selector-limited {
  width: 400px;
  padding: 0 10px;
  margin-bottom: 5px;
  .btn-primary {
    margin-left: 0.5em;
    padding: 3px 6px 2px;
    font-size: $font-down-1;
    min-height: unset;
  }
}