How to set max YouTube size in a chat?

Add maximum size of a YouTube video in a chat. Clicking on a small image will open it in the original size.

.message-text .youtube-wrap {
max-width: 300px !important;
max-height: 300px;
  padding: 0;
height: auto;
}
.youtube-wrap > iframe {
height: auto;
position: static;
}

To know how to edit web CSS, click this link.