Have you read Mongo’s official guide to changing the size of your oplog http://docs.mongodb.org/manual/tutorial/change-oplog-size/ and found it a bit intimidating? Are you resizing it because you already have replication problems anyway? Might as well rebuild your secondary and increase the oplog size in one shot. It’s basically the same as the procedure outlined here: http://docs.mongodb.org/manual/tutorial/resync-replica-set-member/#automatically-sync-a-member.
1. Set the oplog size in /etc/mongodb.conf. Just add this line (the size is in MB):
oplogSize = 102400
2. Stop your server with:
db.shutdownServer()
3. Empty the data directory – the one set by dbPath. You can remove all the files but it’s probably a better idea to move them to a backup directory in case anything goes wrong.
4. Start your server.
That’s it. The server will find the empty data directory, initialize it using the new oplog size, rejoin the replica set and perform a complete initial sync.
Now, you wouldn’t want to do this on a huge production database but if your database isn’t too large, it’ll save you a bit of reading.