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.
Login in CometChat Admin Panel
From left side menu, click on "Settings->Clean Up"
Select Option as per your requirement. such as "Run Individual Clean Up" or "Run Entire Clean Up"
Click on "Run".
"Clear Cache" from left menu
"Clear Browser Cache"