You can change the height of the chat window freely.
Here is how you do it.
(1) Click on the setting button on the upper right of the management page.
(2) Click on [Chat]>[Design]>[Template] on the left side of the screen.
(3) In the section [Height], you can set the height of the chat window.
Click either [Designate in %] or [Designate in px].
※If you designate it in %, enter value within 0-100.
(4) Click on [Update].