mongodbGlobalStorageSize.js

var storageSize = 0
db._adminCommand("listDatabases").databases.forEach(function (d) {
   mdb = db.getSiblingDB(d.name);
   mdb.getCollectionNames().forEach(function(c) {
      s = mdb[c].stats();
      storageSize += s.storageSize
   })
})
printjson((storageSize / 1024 / 1024 / 1024) + "GB")