@ECHO OFF
SETLOCAL enabledelayedexpansion

SET "CHARS=a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,w,w,x,y,z"
SET "INI=0"
SET /P LONGITUD=Inserte la longitud deseada:

CALL :GENERAR

PAUSE
EXIT

:GENERAR
IF %INI% neq %LONGITUD% (
SET /a "INI+=1"
FOR %%c IN (!CHARS!) DO CALL :GENERAR "%~1%%~c"
SET /a "INI-=1"
) else (
SET "CADENA=%~1"
ECHO.!CADENA!
)
GOTO:EOF