In order for you to increase the height of the chat box,
As per deleting the message you can refer CometChat "Clean Up" Functionality which will delete Clean Up All Media & files, Clean Up Old Messages And Inactive Guest Entries, Clean Up Inactive Groups.
Please follow the steps for the same.
STEP 1:
Login in CometChat Admin Panel
STEP 2:
From left side menu, click on "Settings->Clean Up"
STEP 3:
Select Option as per your requirement. such as "Run Individual Clean Up" or "Run Entire Clean Up"
STEP 4:
Click on "Run".
Step 5:
"Clear Cache" from left menu
Step 6:
"Clear Browser Cache"