threema-web/src/sass/components/_emoji.scss

$emoji-height-normal: 1em;
$emoji-height-large: 32px;

img.em {
    height: $emoji-height-normal;
    width: $emoji-height-normal;
    margin: 0 .05em 0 .1em;
    vertical-align: -0.1em;
}

#conversation .message-text {
    .em.large-emoji {
        height: $emoji-height-large;
        width: $emoji-height-large;
        margin: 1px 4px 1px 0px;
    }
}