The amount of space available for an WAS deployed appliance is directly related to the AWS instance type you choose
We have a xref of supported platforms
here is a link that I am referring to -
https://docs.imperva.com/bundle/v13.5-waf-on-amazon-aws-byol-installation-guide/page/10452.htmAs far as your current situation check the /var/tmp and /tmp directories
if you have any .tmp, export, or misc, files you can delete
If you are having trouble you will probably need to open a case.
The long term solution is to deploy on a larger instance