You simply specify the number of OCPUs and the storage capacity in TB's for the database. At any time, you may scale, increase or decrease either the OCPUs or the storage capacity.
https://docs.oracle.com/en/cloud/paas/autonomous-data-warehouse-cloud/user/autonomous-add-resources.html#GUID-DA72422A-5A70-42FA-A363-AB269600D4B0