The free tier lets you make up to 3 vms of the smallest type. So yes, the shared-cpu-256mb ones.
And it includes up to 3GB of storage.
So if you make a development database using 256mb, that would use 1/3 of your 256mb vm allowance. And when it asks how big to make the volume, choose either 1/2/3 GB. As you have a limit of 3GB of storage for free.
You then have two out of your three vm allowance remaining. And so you could deploy an app using one shared-cpu-256mb vm. I think that is the default.
And you would not want to enable any scaling/auto-scaling (well beyond a max of two vms) for that app. Else it could exceed the total free allowance (of three vms). So resources (apart from bandwidth) are automatically limited (by the fact you have not assigned them to be used).
Migrating an app from a free Heroku dyno, I’m very happy with the fly services and dashboard but quite disappointed by the free tier offer: it’s so obvious that most people need one 512mb VM more than three 256mb VMs…