In order for you to embed chat with custom height and width.
Step 1:
Login to CometChat admin panel.
Step 2:
From Left Menu Click on "Settings->Install CometChat"
Step 3:
Search for "Embedded Layout"
Step 4:
Click on "Add To Site" Button.
Step 5:
"HTML Code" Popup appear.
Step 6:
Change "Width" & "Height"
Step 7:
Click on "Generate Code"