[check for filesize]
IF %file% [filesize thing Bigger than] GOTO No
echo Great! Your filesize is smaller than %somany% kbytes.
pause
exit
:no
echo Um... You have a big filesize.
pause
exit

@echo off
setlocal
set file="test.cmd"
set maxbytesize=1000

FOR /F "usebackq" %%A IN ('%file%') DO set size=%%~zA

if %size% LSS %maxbytesize% (
    echo.File is ^< %maxbytesize% bytes
) ELSE (
    echo.File is ^>= %maxbytesize% bytes
)

C:> call /?

C:> if /?

@ECHO OFF
SET SIZELIMIT=1000
SET FILESIZE=%~z1

IF %FILESIZE% GTR %SIZELIMIT% Goto No

ECHO Great! Your filesize is smaller than %SIZELIMIT% kbytes.
PAUSE
GOTO :EOF

:No
ECHO Um ... You have a big filesize.
PAUSE
GOTO :EOF

@echo %~z1

Set objFS = CreateObject("Scripting.FileSystemObject")
Set wshArgs = WScript.Arguments
strFile = wshArgs(0)
WScript.Echo objFS.GetFile(strFile).Size & " bytes"

C:test>cscript /nologo filesize.vbs file.txt
79 bytes

>dir thing