Of course it can be less than this, but it is pretty much enough to start to organize your Slackware installation exactly as you want it. You can even reuse some old memory stick. I prefer to have usb stick much more than running small cd distributions, such as "Damn Small Linux" because it is writable and you can update it easily. An interesting package is slapt-get that enables to load packages and all dependencies from the net.
but, here is what you have to install (for the minimal Slackware installation):
You can install packages from a working Slackware installation with:
# installpkg -root /wherewewanttoinstall name(s)ofpackage(s).tgz
Or you can boot to Slackware DVD, mount the DVD, mount the partition, install packages from the DVD with the same command.
chroot to the partition with the installation,
(find somewhere how to do it)
# mount -o bind /dev /wherewewanttoinstall/dev
# mount -t proc /proc /wherewewanttoinstall/proc
Reboot and boot your new (light) Slackware installation and customize it as you wish. It is about 170M. I used it on old computers that cannot boot from usb, to boot from hard drive, then continue with usb stick. even 266Mhz PentiumII machine works very fine with it.