Mongo shell script to scan database sizes.

gistfile1.js

var s = {}
var d = db.getMongo()
var dbnames = d.getDBNames()

for (var i = 0; i < dbnames.length; ++i) {
	var dbh = d.getDB(dbnames[i])
	var cnames = dbh.getCollectionNames()

	s[dbnames[i]] = 0
	//print("Scanning: " + dbnames[i])

	for (var j = 0; j < cnames.length; ++j) {
		//print("Scanning: " + dbnames[i] + "." + cnames[j])
		s[dbnames[i]] += dbh.getCollection(cnames[j]).dataSize()
	}
}

var total = 0

for (var k in s) {
	total += s[k]
	print(k + "\t" + s[k])
}

print("total\t" + total)