custom.css

:root {
    --default-emoji-size: 1.375rem;
    --default-emoji-jumboable-size: 3rem;
    --emoji-size: calc(var(--default-emoji-size) * 2);
    --emoji-jumboable-size: calc(var(--default-emoji-jumboable-size) * 2);
}

/* Change emoji size */
[class*="messageContent"]:not([class*="repliedTextContent"]) .emoji {
    height: var(--emoji-size) !important;
    width: var(--emoji-size) !important;
}

/* Change emoji size when jumboified */
[class*="messageContent"]:not([class*="repliedTextContent"]) .jumboable {
    height: var(--emoji-jumboable-size) !important;
    width: var(--emoji-jumboable-size) !important;
}