converse.js/sass/converse/_chatrooms.scss

#conversejs {
    .chatbox {
        &.chatroom {
            min-width: $chatroom-width !important;
            width: $chatroom-width;
            .box-flyout {
                min-width: $chatroom-width !important;
                width: $chatroom-width;
            }

            .chatroom-body {
                .occupants {
                    .chatroom-features {
                        .feature {
                            font-size: $font-size-small;
                        }
                    }
                }
            }
        }
    }
}