ok, Ill explain here
dir /b /o-s>{temp}
create a directory listing of the directory, the /b means just the filename, the /o-s means sort the list with the biggest first
now send the list into a file called {temp} ....the curly brackets makes these files stand out in a directory listing
set /P biggest=<{temp}
set creates a variable, the /P means wait for the user to type something and press enter, BUT we are redirecting the input from a file, so it will start to read the temporary file, until it hits the first carriage return, when it will stop, the variable %biggest% now holds the first line of the file (which is why we sorted the listing biggest to smallest)
and thats it