How much do I get by default, and how much can I add?
A Platform.sh Professional project comes with 5 GB of storage per environment. You can allocate that space between your application container and each of your services as you like, using the disk property in services.yaml and .platform.app.yaml.
You can increase the per-environment disk space yourself through the UI, up to 50 GB per environment. The price is based on the disk space and number of environments you have on your plan (which you can also adjust yourself.) If you want more than 50 GB please file a support ticket so we can increase it for you. It’s not a problem, we just need to make sure the grid host you’re on has sufficient space for it.
Platform.sh Enterprise comes with 50 GB of storage for the staging and production environments. That can be increased (but not decreased) by filing a support ticket.
Cheers.