Note: This information is for our self-hosted server product and is unrelated to CircleCI cloud
CircleCI server customers can use any resource class they want without any restrictions or approval since they are using their infrastructure.
Customers can also customize resource classes for their installation to provide developers with CPU/RAM options for the Jobs they configure. Check this document to know how to customize the resource class.
Note: Container size (resources) cannot be more than that of the underlying nomad clients. In such a situation, that particular job will always just remain queued.