How to block large files to being pushed to Codecommit

To implement this, you need a server-side pre-receive git hook. Currently, this feature is not supported by AWS CodeCommit. However, there is already a feature request in our backlog, so the service team is aware of that. Unfortunately, I have no further information on when or whether this feature will be released. I encourage you to periodically check the What's New page and look for the announcement.

Until then, you can consider implementing an AWS Lambda function that will be triggered on pushing new code to a CodeCommit repo. This function could check the content of the new commits and take some actions on that, e.g. notify the committer. While it can't prevent such commits from being stored in the repository, you will still have a mechanism to react on pushing large files.