StrLen.bat

setlocal
set "myString=abcdef!%%^^()^!"
call :strlen result myString
echo %result%
goto :eof

:strlen <resultVar> <stringVar>
(   
    setlocal EnableDelayedExpansion
    set "s=!%~2!#"
    set "len=0"
    for %%P in (4096 2048 1024 512 256 128 64 32 16 8 4 2 1) do (
        if "!s:~%%P,1!" NEQ "" ( 
            set /a "len+=%%P"
            set "s=!s:~%%P!"
        )
    )
)
( 
    endlocal
    set "%~1=%len%"
    exit /b
)

StrLen2.bat

:: Returns the string length.
:: PARAM
:: 1 String string to get length of
:: RETURN Integer string length
:STRLEN
SETLOCAL ENABLEEXTENSIONS ENABLEDELAYEDEXPANSION
SET STR="%~1"& SET STR=!STR:~1,-1!
SET LEN=0
:: Max string length is 8191, see http://support.microsoft.com/kb/830473
SET POS=8192
:STRLEN_LOOP
SET /A POS /= 2
IF NOT "!STR!" == "" (
    IF NOT "!STR:~%POS%,1!" == "" (
        SET /A LEN += %POS% + 1
        :: work on rest, that is after POS+1
        SET STR=!STR:~%POS%!
        SET STR=!STR:~1!
    )
    GOTO :STRLEN_LOOP
)
ENDLOCAL & SET RESULT=%LEN%
GOTO :EOF