Deploy a test instance and find out? I think, 4GB images are fine: Fly Machines API - Create Machine from Private Container Registry - #4 by ignoramous
As for us, we do bake-in some ~100MB files in the docker image itself: Registry @ registry.fly.io, is there cost to this? - #2 by ignoramous
Not in this case, really.
Every Fly app instance comes with a built-in ephemeral disk space 8GB in size. You can opt to either sftp the db files to those VMs one by one, or download them (from S3 / R2) when the app process is run for the first-time and/or the db files at an expected location (somewhere in / (root dir)) are missing.