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 mongo = db.getMongo();

function bytesToSize(bytes) {
   var sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB'];
   if (bytes == 0) return '0 Byte';
   var i = parseInt(Math.floor(Math.log(bytes) / Math.log(1024)));
   return Math.round(bytes / Math.pow(1024, i), 2) + ' ' + sizes[i];
}

function getCollectionStatsFor(dbName, sort) {
    var db = mongo.getDB(dbName);
    db.getCollectionNames()
        .map(x => db.getCollection(x).stats())
        .sort((a, b) => sort ? b.size - a.size : 0)
        .map(x => ({ name: x.ns, size: bytesToSize(x.size), storageSize: bytesToSize(x.storageSize) }))
        .forEach(x => print(`${x.name}: ${x.size} (${x.storageSize})`));
}

function getDatabasesStats(sortDatabases, sortCollections) {
    mongo.getDBNames()
        .map(x => mongo.getDB(x).stats())
        .sort((a, b) => sortDatabases ? b.dataSize - a.dataSize : 0)
        .map(x => ({ name: x.db, size: bytesToSize(x.dataSize), storageSize: bytesToSize(x.storageSize) }))
        .forEach(x => {
            print(`\n    ${x.name}: ${x.size} (${x.storageSize})\n`);
            getCollectionStatsFor(x.name, sortCollections);
        });
}

getDatabasesStats()