Quick list the size of a dir tree and contained files in mbytes with a free open source command line tool for Windows, Mac OS X and Linux.

sfk stat [-minsize=n] dir [-i]
sfk treesize[mb] dir

show directory tree size statistics in mbytes.
hidden and system files are INCLUDED by DEFAULT,
to make sure the real size usage is always shown.

options
   -minsize=n   list only dirs and files with a size larger
                or equal to this, e.g. 100 = 100 bytes,
                10k = 10 kbytes, 10m = 10 megabytes.
   -i           read list of directories from stdin.
   -nohidden    exclude hidden and system files.
   -nosum       print no summary at end of listing.
   -sum         print only the summary.
   -kbytes      list all sizes in kbytes.
   -bytes       list all sizes in bytes.
   -flat        show the no. of files and bytes per folder
                without its subfolders (do not accumulate).

aliases
   sfk stat10       does the same as sfk stat -minsize=10m
   sfk treesize100  does the same as sfk stat -minsize=100m

limitations
   this command cannot summarize zip file contents.

web reference
   http://stahlworks.com/sfk-stat

examples
   sfk stat . -minsize=10m
       list sizes of all directories below the current one
       having a size of at least 10 mbytes.

   sfk stat50 docs
       list all directories and single files under the docs
       directory tree having a size of >= 50 mbytes.

   type dirlist.txt | sfk stat -quiet -i
       list sizes of dirs listed in dirlist.txt
 
example with output:

   sfk stat -minsize=5 mozilla

     -> lists size stat for directory mozilla.
     -> within this dir, lists only objects >= 5 mbytes,
        providing a quick info about the largest things.

   result:

       7 mb,  1456 files,  mozilla\browser
      10 mb,  2234 files,  mozilla\content
       6 mb,   195 files,  mozilla\db
      11 mb,  2182 files,    mozilla\dom\tests\mochitest
      11 mb,  2224 files,   mozilla\dom\tests
      13 mb,  2980 files,  mozilla\dom
       5 mb,  1418 files,  mozilla\embedding
      15 mb,  3880 files,  mozilla\extensions
       7 mb,   713 files,  mozilla\gfx
       8 mb,   865 files,   mozilla\intl\uconv
      10 mb,  1375 files,  mozilla\intl
       7 mb,   657 files,   mozilla\js\src
       7 mb,   705 files,  mozilla\js
       6 mb,  2834 files,    mozilla\layout\html\tests
       6 mb,  2838 files,   mozilla\layout\html
      21 mb,  7636 files,  mozilla\layout
       7 mb,   396 files,     mozilla\modules\libpr0n\test\reftest
       7 mb,   425 files,    mozilla\modules\libpr0n\test
       8 mb,   602 files,   mozilla\modules\libpr0n
      18 mb,  2664 files,  mozilla\modules
       5 mb,   977 files,  mozilla\netwerk
       5 mb,   642 files,   mozilla\nsprpub\pr
       6 mb,   877 files,  mozilla\nsprpub
       5 mb,    60 files,    mozilla\plugin\oji\JEP
       7 mb,   306 files,   mozilla\plugin\oji
       7 mb,   311 files,  mozilla\plugin
      19 mb,  1189 files,    mozilla\security\nss\lib
      25 mb,  2735 files,   mozilla\security\nss
      28 mb,  3282 files,  mozilla\security
       5 mb,  1254 files,   mozilla\toolkit\components
      14 mb,  3843 files,  mozilla\toolkit
       7 mb,  1159 files,  mozilla\xpcom
     228 mb, 40864 files, mozilla
     228 mb, 40864 files, 5280 dirs, 228130292 bytes.

save typing by aliases:

   if you often use a command like "sfk stat -minsize=5 ...",
   you may create an sfk alias, for example:

      sfk alias stat sfk stat -minsize=%1

   the above command creates a batch file "stat.bat" in your path.
   further parameters passed to this batch are appended automatically.
   now, to do the same as in the mozilla example above, just type

      stat 5 mozilla