static/app.css

#maiChatHistory .message-wrap div {
    font-size: 30px;
}

#maiChatInput textarea {
    font-size: 30px
}

@media screen and (min-width: 768px) {
    #maiChatInput textarea {
        font-size: 20px
    }

    #maiChatHistory .message-wrap div {
        font-size: 20px;
    }
}