Use to calculate sizes of MongoDB collections.

mongodb_collection_sizes.js

use <dbname>;

var collectionNames = db.getCollectionNames(), stats = [];

collectionNames.forEach(function(n){ 
    stats.push(db[n].stats()); 
});

stats = stats.sort(function(a, b) { return b['size'] - a['size']; });

for(var c in stats){ 
    print(stats[c]['ns'] + ": " + stats[c]['size'] + " bytes (" + stats[c]['storageSize'] + " bytes)"); // in bytes 
    //print(stats[c]['ns'] + ": " + stats[c]['size']/1000000 + " MB (" + stats[c]['storageSize']/1000000 + " MB)"); // in MBs
    //print(stats[c]['ns'] + ": " + stats[c]['size']/1000000000 + " GB (" + stats[c]['storageSize']/1000000000 + " GB)"); // in GBs
}