General questions regarding RDS SQL Server Read Replicas

No. Read replicas for RDS SQL Server are implemented using the built-in Distributed Availability Groups feature in SQL Server. Read replcias do not use the replication feature provided by Microsoft SQL Server.

Read replicas have the same size limitations as the primary instance in terms of size. A read replica is created from a snapshot of the source instance upon creation and after that it's synchronized asynchronously. Recommendation is to make sure that the source DB instance and its read replicas are sized properly, in terms of computing power and storage, to suit their operational load. As long as you have comparable instance type/size (RAM, CPU) and storage type (EBS volume type and IOPs), synchronization should be smooth. Similar note in the documentation here.

Note that read replicas use asynchronous data movement from the source. So make sure read replicas have similar resources to the primary instance so they can catchup with the rate of change on the primary.