Starting today, Amazon RDS for SQL Server db.m5 and db.r5 instance classes are available in 8xlarge and 16xlarge sizes. With support for these new instance sizes, customers who are currently using either m4.10xlarge, m4.16xlarge, r4.8xlarge, or r4.16xlarge now have an easy upgrade path to the latest generation of instances.  

from Recent Announcements: https://aws.amazon.com/about-aws/whats-new/2019/11/amazon-rds-for-sql-server-now-supports-additional-instance-sizes/