If you install linux first and then windows, windows will overwrite your boot sector and you will have to boot to linux with a floppy disk, rewrite your boot file (lilo.conf or whatever you choose to use) to include both windows installs, and then rewrite the boot sector. If you install linux last, it will take care of all of that for you.
Partition size depends on how much space you're willing to give your linux install. 4 gigs should be more than enough if you have that much. Don't forget that you'll need to make a swap partition for your linux os.
ls will list directories.
I believe that dir is aliased to ls by default in mandrake
ls -a will show all hidden files (files beginning with .)
ls -l will show file properties.
ls -al will show both.
Those are the most commonly used ls switches. Good luck.