You can change the height of the chat window freely.
Here is how you do it.

(1)Click the setting button on the upper right of the management page.

(2)Click [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 [Update]