There is no “capacity” attribute for storage, but there is a “minimum-size”
At deploy time, the user can then (or is expected to in production scenarios) specify the actual storage configuration to be used, eg
$ juju deploy foo --storage database=100G
The default storage provisioned on k8s is a PV but you can also choose to use k8s emptyDir backed by either memory or disk for prototyping or testing etc, eg
$ juju deploy foo --storage database=tmpfs,1G
$ juju deploy foo --storage database=rootfs,1G
rootfs storage types map to generic Juju storage concepts applicable to any type of deployment, k8s or cloud.
See Persistent storage and Kubernetes for more info.