Pool size options can be specified as part of the connection string. The options are listed in the MongoDB documentation. Connection string parameters are mentioned in the MongoJS documentation but it's not particularly obvious that one would find the answer there.
var db = mongojs('mongodb://localhost/mydb?maxPoolSize=200');
A minPoolSize
can also be specified, however this option is not supported by all drivers. I am pretty sure that the native js driver does not support this option. From memory the native js driver keeps a minimum pool of 5 connections.
The default maxPoolSize
value is 100.