Welcome to LQ.
A full install of slackware will put around 6.5 - 7 GB of stuff in /usr and about 512M of stuff in /. You probably want to allow some extra for growth and adding new software though.
The rest is really down to personal preference and will depend on how you intend to use the space.
If all you want is a '/' and '/home' then a 16GB '/' is probably not a bad number. As tinkster said, you'd probably get away with 10GB, but you might start to find it gets a bit tight.
Last edited by GazL; 07-01-2010 at 05:06 PM.