Do AWS RDS cluster instances need to be the same size?

I have a 2 instance RDS Aurora cluster (one writer and one reader). Is there any reason my reader instances need to be the same size as the writer?

Is there any problem creating a cluster with a db.r3.4xlarge (writer) and a db.r3.2xlarge (reader)?

My writer uses a lot more CPU load than my reader so I want to scale the reader down so it's not just wasting money. It never even uses 1/2 the CPU as the writer. Any issues with this? I looked through the AWS documentation but I can't find any mention of it.

Yes, you can use it, but there can be two potential issues to consider:

1. Your replica may not keep up with the writer. You already observed that that replicas use much as resources than writer so this shouldn't be a problem.
2. If writer fails, your replica will become new writer. Thus if your writer must support heavy load, the new writer, on smaller instance, my under-perform.