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'] + ")"); }

If you're using a newer version of mongo which supports ES6 this is a more terse form of the above with information about the index sizes and uses 3 decimal places like show dbs does from the mongo shell

const readableSize = (size) => {                                               
  const scale = (Math.log(size) / Math.log(1024)) | 0;                         
  return (size / Math.pow(1024, scale)).toFixed(3) + ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'][scale];
}                                                                              
                                                                               
const printStats = (mongo) => {                                                
  mongo.getDBNames()                                                           
    .map((i) => mongo.getDB(i))                                                
    .forEach((db) => {                                                         
      print(db);                                                               
      print(Array(81).join('='));                                              
      db.getCollectionNames()                                                  
        .map((i) => db.getCollection(i).stats())                               
        .sort((a, b) => b.size - a.size)                                       
        .forEach((s) => {                                                      
          print(`${s.ns}: ${readableSize(s.size)} (${readableSize(s.storageSize)})`);
          print(`Indexes: (${readableSize(s.totalIndexSize)})`);               
          Object.entries({...s.indexSizes}).sort((a, b) => b[1] - a[1]).forEach(([n, s]) => {
            print(`  ${n}: ${readableSize(s)}`);                               
          });                                                                  
          print('');                                                           
        });                                                                    
                                                                               
      print('');                                                               
    });                                                                        
}                                                                              

// and used:                                                                               
printStats(db.getMongo())                                                      

which would render:

admin
================================================================================
admin.system.users: 4.284kB (36.000kB)
Indexes: (72.000kB)
  _id_: 36.000kB
  user_1_db_1: 36.000kB

admin.system.keys: 425.000B (36.000kB)
Indexes: (36.000kB)
  _id_: 36.000kB

admin.system.version: 104.000B (36.000kB)
Indexes: (32.000kB)
  _id_: 32.000kB


config
================================================================================
config.system.sessions: 41.995MB (9.809MB)
Indexes: (38.055MB)
  _id_: 36.574MB
  lsidTTLIndex: 1.480MB

config.transactions: 304.604kB (204.000kB)
Indexes: (1.250MB)
  _id_: 1.250MB

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.