This snippet walks through your mailbox and tallies message sizes, flagged and unread status using the Python Scripting Bridge and Mail.app. It isn't speedy, but it appears to be light on system resources.

inbox.py

from ScriptingBridge import *
import os, sys, re

pattern = re.compile("^(INBOX|Sent*|Draft*)")

class Census:
  """Tallies mail info"""
  def __init__(self, pattern):
    self.app = SBApplication.applicationWithBundleIdentifier_("com.apple.mail")
    self.accounts = dict([(a.name(),a) for a in self.app.accounts()])
    # Jedi mind tricks
    self.mailboxes = reduce(list.__add__,list([[m for m in a.mailboxes() if pattern.match(m.name())] for a in self.app.accounts()]))
    self.pattern = pattern
    self.stats = {'unread':0, 'flagged':0, 'total':0, 'bytes':0}

  def run(self):
    for m in self.mailboxes:
      print m.name()
      self.stats['unread'] += m.unreadCount()
      for i in m.messages():
        print i.subject()
        if i.flaggedStatus():
          self.stats['flagged'] += 1
        self.stats['total'] += 1
        self.stats['bytes'] += i.messageSize()
    return self.stats
        
if __name__ == '__main__':
  c = Census(pattern)
  print c.run()

First step towards a bayesian filter on Mail.app?