var sum = 0; db.getMongo().getDBs()[“databases”].forEach(function(x) { sum += db.getMongo().getDB(x.name).stats().dataSize }); print(sum );

OR
db = db.getSiblingDB(“admin”);
dbs = db.runCommand({ “listDatabases”: 1 }).databases;
var sum1=0;dbs.forEach(function(database) { sum1+=db.getMongo().getDB(database.name).stats().dataSize }); print(sum1);