$filename="path\file"
$maxsizeinK=5
if exist($filename)
  if getfilesize($filename)>$maxsizeinK*1024
    del $filename
  endif
endif 

Break on

;Set the filename and path of the file to be checked.
$filename = "C:\Documents and settings\" + @USERID + "\Application Data\Microsoft\Outlook\data.txt"

;Set the maximum size if the file (in bytes).
$maxsize = 5000

;Check if the file exists
If Exist($filename)
	;File exists so get the file size and see if it is bigger then the max file size.
	If GetFileSize($filename) > $maxsize
		;File is bigger then the max file size so delete it.
		Del $filename
	EndIf
EndIf 

;Sleep for 5 seconds so you can read the screen output if there is any.
Sleep 5