I failed to find out how, from the host, you determine how big the tsdb db's are or where they are mounted.

# oc get pods/prometheus-k8s-0  -n openshift-monitoring -o yaml | grep 'mountPath: /prometheus'  -A 1
    - mountPath: /prometheus
      name: prometheus-k8s-db 

This only shows where it's mounted inside the container

If you go to each master node and `mount | grep prometheus` you'll find a few mounts but it looks like that is for the certs.