As of 4.3.4, replSetResizeOplog requires setting the oplog size when configuring minRetentionHours. This is a bit unusual, as it requires the user to retrieve the current oplog size before changing the minimum retention period if they only want to modify the retention period.

This behavior is not reflected in the commandline or configuration file options, where oplogSize or replication.oplogSizeMB are not dependencies.

It looks like design decision happened as part of a discussion during code review:

geert 2020/02/14 18:06:42
We'll always require a size. This is required by capped collections. That
requirement is mostly historical, but not something we should change in this
patch. Not requiring a size would lead to downgrade issues, and would turn into
significant extra work.

As per the review for DOCSP-8653, revisit this discussion and see if this is something that can be fixed while addressing the original downgrade concerns.