We get a lot of MMSSUPPORT tickets asking about or requesting oplog size increases. Customers cannot do this. It requires a HELP support ticket and a HELP request.
I think we should mention in the ATLAS FAQs that a customer cannot resize oplogs and possibly that should more oplog be required that they open a ticket with some details like:

If you would like MongoDB Atlas to increase the size of the oplog please provide the following:
▪ The specific Atlas cluster you are looking to increase the oplog of (as well as the cluster's URL).
▪ The intended oplog size you would like to see for the deployment.
▪ The use case requiring the additional oplog as we may be able to accommodate the use case with the current oplog.