Hi @Joanne,
Reducing the size of the oplog will reduce the amount of data it can hold. You want your oplog to be able to hold at least as much data as your longest potential downtime would be. If you have some sort of monitoring on your system you will be able to trend how much of a window your oplog contains over time. Reducing the oplog size could cause problems with a resync if too small as the oldest data could be overwritten before the sync completes and you would be stuck in a loop of trying to sync data.
My recommendation is to keep the oplogs the same size on all machines in a replica set. You never know when your primary will go down and if the secondary has a smaller oplog size t will hold less data which could cause problems. Also note that all the data in the Primary oplog is replicated down to the Secondary oplogs so your Secondary oplog window would be shorter.