As per the documentation[0],

[0] https://docs.openshift.com/container-platform/4.15/observability/logging/log_storage/cluster-logging-loki.html#:~:text=anti%2Daffinity%20rules-,Zone%20aware%20data%20replication,-In%20the%20logging

When configuring the deployment size as 1x.extra.small , receiving below error message: 

$ oc apply -f lokistack
The LokiStack "logging-loki" is invalid: spec.size: Unsupported value: "1x.extra.small": supported values: "1x.demo", "1x.extra-small", "1x.small", "1x.medium"

Document bug opened to correct the deployment size in OCP version 4.16 document and lower.