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 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'] + " (" + stats[c]['storageSize'] + ")"); }

Try this to get all DBs, collections and document count:

mongo --eval " db.getMongo().getDBNames().forEach( function(v, i){ db.getSiblingDB(v).getCollectionNames().forEach( function(vv, ii){ print (v + ',' + vv + ',' + db.getSiblingDB(v).getCollection(vv).count() ) } ) } ) "

Is there a way to then store this data in a collection as historical data . To monitor collection size growth over a period of time

@nenohweg something like datadog?

Also here's a version which works better for me - the versions at the top of this thread *** will silently omit values as they error on system.profile ***

(function() {
  const byteUnits = [" kB", " MB", " GB", " TB", "PB", "EB", "ZB", "YB"];

  // get collections by size sorted
  function getReadableFileSizeString(fileSizeInBytes) {
    let i = -1;
    do {
      fileSizeInBytes = fileSizeInBytes / 1024;
      i++;
    } while (fileSizeInBytes > 1024);

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

  const collectionNames = db.getCollectionNames();
  let stats = [];
  for (const name of collectionNames) {
    if (name === 'system.profile') {
      continue;
    }
    stats.push(db.getCollection(name).stats());
  }
  stats = stats.sort(function(a, b) {
    return b["size"] - a["size"];
  });
  for (let i = 0; i < stats.length; i++) {
    console.log(stats[i]["ns"] + ": " + getReadableFileSizeString(stats[i]["size"]) + " (" + getReadableFileSizeString(stats[i]["storageSize"]) + ") - ", Number(stats[i].count).toLocaleString(), " docs");
  }
})();

Also includes formatted doc count.