Get all collection size:

collectionNames.forEach(function (n) { stats.push(db[n].stats()); });
var byteToMB = 1024*1024;
for (var c in stats) {     print(stats[c]['ns'] + ": " + stats[c]['size']/byteToMB + "(MB) (" + stats[c]['storageSize']/byteToMB + ") (MB)"); }