For a pair of days I reinstalled Debian Testing with Fluxbox and elected to use only 5 GB to do my day to day tasks. This included my entire root partition (though not swap) and user directory, which I tend not to put on a separate partition because I don't want to mess up each OS' configuration files.
The OS, after most of my required programs were in place, was a surprisingly hefty 1.8 GB. I dumped a bit of music in there and it became rather crowded.
I will be honest: it was difficult. I'm used to having room for downloads and larger files. However, it was rather enlightening because I had to keep track of my space, and this, keep things neat and tidy.
I can't do it all day, every day. I've already went back to my main Ubuntu install; too many things I use or need on a regular basis. Besides, I do have backups of my most needed documents and files, but it's not a backup if it's my only copy, right? As much as I'd like to, I can't really squish myself into 5 GB.
Enjoyable experiment, though. Give it a shot if you have a few gigs to spare.