var dbs_names = db.adminCommand('listDatabases');
for (var i = 0; i < dbs_names.databases.length; i += 1) {
    var current_db = connect(dbs_names.databases[i].name);
    var collection_names = current_db.getCollectionNames();
    for (var j = 0; j < collection_names.length; j += 1) {
        var collection = current_db[collection_names[j]];
        var size = collection.stats().count;
        print(dbs_names.databases[i].name + '\t' + collection_names[j] + ':\t' + size);
    };
};