List mongodb collections in descending order of size. Helpful for finding largest collections. First number is "size," second is "storageSize."

mongodb_collection_sizes.js

var mgo = db.getMongo()

function getReadableFileSizeString(fileSizeInBytes) {

    var i = -1;
    var byteUnits = [' kB', ' MB', ' GB', ' TB', 'PB', 'EB', 'ZB', 'YB'];
    do {
        fileSizeInBytes = fileSizeInBytes / 1024;
        i++;
    } while (fileSizeInBytes > 1024);

    return Math.max(fileSizeInBytes, 0.1).toFixed(1) + byteUnits[i];
};

function getStatsFor(db){
    var collectionNames = db.getCollectionNames(), stats = [];
    collectionNames.forEach(function (n) { stats.push(db.getCollection(n).stats()); });
    stats = stats.sort(function(a, b) { return b['size'] - a['size']; });
    for (var c in stats) { print(stats[c]['ns'] + ": " + getReadableFileSizeString(stats[c]['size']) + " (" + getReadableFileSizeString(stats[c]['storageSize']) + ")"); }
}

function getAllStats(){
    mgo.getDBNames().forEach(function(name){ var db = mgo.getDB(name); print('\n    '+db+'\n'); getStatsFor(db) })
}

getAllStats()