Wow thanks westfw and all others posted here!
For now, I used the temporary command:
path C:\Users\Federico\Desktop\arduino-0021\hardware\tools\avr\bin;%path%
to add the executable to the windows path, then move into the compiled sketch folder and launch the command:
...>cd C:\Users\Federico\AppData\Local\Temp\build4288029111388491449.tmp
C:\Users\Federico\AppData\Local\Temp\build4288029111388491449.tmp>avr-size SketchName.cpp.elf
And it works well!!
Now i need try to find the "autoexec.bat" and add the path there...