I solved similar problem. Some of methods in this page are slow and some are problematic in multilanguage environment (all suppose english). I found simple workaround using vbscript in cmd. It is tested in W2012R2 and W7.
>%TEMP%\_SFSTMP$.VBS ECHO/Set objFSO = CreateObject("Scripting.FileSystemObject"):Set objFolder = objFSO.GetFolder(%1):WScript.Echo objFolder.Size
FOR /F %%? IN ('CSCRIPT //NOLOGO %TEMP%\_SFSTMP$.VBS') DO (SET "S_=%%?"&&(DEL %TEMP%\_SFSTMP$.VBS))
It set environment variable S_. You can, of course, change last line to directly display result to e.g.
FOR /F %%? IN ('CSCRIPT //NOLOGO %TEMP%\_SFSTMP$.VBS') DO (ECHO "Size of %1 is %%?"&&(DEL %TEMP%\_SFSTMP$.VBS))
You can use it as subroutine or as standlone cmd. Parameter is name of tested folder closed in quotes.