The AWS docs recommend verifying there is enough capacity for the replication overhead.

Tasks:

• Verify that the blue instance has adequate capacity by checking the following metrics:
  • FreeStorageSpace
  • TransactionLogsGeneration
  • TransactionLogsDiskUsage
  • OldestReplicationSlotLag

To estimate the additional storage required on the blue instance, monitor the TransactionLogGeneration CloudWatch metric during peak workload periods. For example, if your workload generates 100 GB of write ahead logs (WAL) data over 24 hours, ensure you have at least 100 GB of extra storage to accommodate one day's worth of WAL segments.

See the graphs here
    
Over the past 6 months, we are generally around 1.5Gb a day, although we have had peaks of up to 4.4Gb in a day.