In order for you to increase the height of the chat box,
You need to login to CometChat Administrative panel.
Browse through Settings on the left pane, go through General section and look for "Docked Layout Settings"
As shown below in screenshot
From there Chat box dimensions can be modified as shown below in screenshot
The height for "Chat Now" tab cannot be modified directly.
You can modify the above with the help of web developer, using custom.css file available under Settings and Editor tab.