FEATURE DESCRIPTION
This new feature enables the dynamic resizing of a node's oplog, allowing users to grow and shrink the oplog to satisfy the operational needs of each node in a replica set.

VERSIONS
This feature is available in the 3.5.10 and newer development versions of MongoDB, and in the 3.6 and newer production releases.

RATIONALE
In a MongoDB replica set, the oplog is a special capped collection used to replicate data to other nodes. Users may specify the size of the oplog for each node when deploying a replica set, but altering that size required maintenance.

This new feature bypasses the need for maintance and allows users to dynamically change the size of the oplog in MongoDB nodes running with the WiredTiger storage engine only (nodes running the MMAPv1 storage engine still need to follow the maintenance procedure linked above).

OPERATION
To display the size of the oplog, one can use the stats() command. Here's a replica set primary node with a 9.1GB oplog:

replset:PRIMARY> use local
replset:PRIMARY> db.oplog.rs.stats()
{
        "ns" : "local.oplog.rs",
        "size" : 6781,
        "count" : 60,
        "avgObjSize" : 113,
        "storageSize" : 36864,
        "capped" : true,
        "max" : -1,
        "maxSize" : NumberLong("9790804377"),
        ...
}

The size of the oplog is displayed at deployment time in the logs as follows:

2017-07-17T17:47:11.870-0400 I REPL     [replication-0] creating replication oplog of size: 9337MB...

Users can change the size of the oplog with the replSetResizeOplog command, specifying a size in MB. For example:

replset:PRIMARY> use admin
replset:PRIMARY> db.runCommand({replSetResizeOplog:1, size: 16384})
{ "ok" : 1, "operationTime" : Timestamp(1500329291, 1) }

The command above changes the oplog size to 16384 MB (16GB/17179869184 bytes). This operation is recorded in the logs as follows:

2017-07-17T17:52:40.396-0400 I STORAGE  [conn11] replSetResizeOplog success, currentSize:17179869184

The output of the stats() command also reflects the change:

replset:PRIMARY> use local
replset:PRIMARY> db.oplog.rs.stats()
{
        "ns" : "local.oplog.rs",
        "size" : 6781,
        "count" : 60,
        "avgObjSize" : 113,
        "storageSize" : 36864,
        "capped" : true,
        "max" : -1, 
        "maxSize" : NumberLong("17179869184")
        ...
}

ADDITIONAL NOTES
Reducing the size of the oplog removes data from it. Reducing the size of the oplog in a given node may cause any members of a replica set syncing from that node to become stale and need a resync.