What are acceptable values for the `--vm-size` option for "fly postgres create"?

I’m trying to follow the instructions here: Flyctl

They give me this info:

  -h, --help                  help for create
      --name string           the name of the new app
      --organization string   the organization that will own the app
      --password string       the superuser password. one will be generated for you if you leave this blank
      --region string         the region to launch the new app in
      --snapshot-id string    Creates the volume with the contents of the snapshot
      --vm-size string        the size of the VM
      --volume-size string    the size in GB for volumes

I tried passing both “256” and “256mb” as the string for --vm-size, but both got rejected with errors like:

Error vm size "256mb" not found

What are valid values that I can pass? And specifically, what is the valid value for the smallest one, because I want to stay on the free tier if possible.

Thanks.

@crowell.jon You can see the available vm-sizes by running:

$ fly platform vm-sizes

NAME             CPU CORES MEMORY
shared-cpu-1x    1         256 MB
dedicated-cpu-1x 1         2 GB
dedicated-cpu-2x 2         4 GB
dedicated-cpu-4x 4         8 GB
dedicated-cpu-8x 8         16 GB