i've always wondered how one changes partition sizes in linux, but could never find out
all i know is that you should definitely leave about 6 gigs to install slackware now, especially if u just want to take the easy step of installing everything (like me)
not exactly sure how u remove specific items, but i can suggest that since this is probably a fresh install, just reinstall but use fdisk to resize your partitions (and partition magic if necessary) and then in the installer select only pacakges that are required, sort through the optional ones, choosing only ones you will probably use (like web-browsing=just mozilla will do, a window manager, a few games, maybe gimp, and whatever else)