discourse/app/assets/stylesheets/common/base/emoji.scss

img.emoji {
  width: 20px;
  height: 20px;
  vertical-align: text-bottom;

  h1 &,
  h2 &,
  h3 &,
  h4 &,
  h5 &,
  h6 & {
    vertical-align: middle;
  }
}

img.emoji.only-emoji {
  width: 32px;
  height: 32px;
  margin: 0.5em 0;
}

a,
.md-table,
.poll {
  img.emoji.only-emoji {
    width: 20px;
    height: 20px;
    margin: 0;
  }
}

small img.emoji,
sub img.emoji,
sup img.emoji {
  height: 1.1em;
  width: 1.1em;
}