I used to have a pretty good HOWTO on installing a minimal Slack system, but don't publish it anymore as it is out of date. Still, I have no problem to install a runnign system with X and a desktop environment in less than 250MB without cramping my style, except for needing compilers, etc. A system which can recompile itself needs round 400-500MB. And these figures mean using regular packages without deleting docs, header files or other non-essentials.
Anyway, a few months spent working on trimming down your system will teach you a great deal about what really makes you system tick.