When I creating client to replica set with MongoClient.connect and pass own value to 'options' parameter 'server.poolSize' (e.g. server.poolSize = 3), this parameter has been overwritten to default value 5. I see it in netstat command and mongod logs.
I found what it has been overwriten in lib/mongodb/connection/repl_set/options.js on line 118, I think here needs something like:
server.poolSize = server.poolSize || self.poolSize;