That all depends on how much software you're gonna be using and how much you wanna get involved with the system. If you don't mind getting your hands dirty with editing config files and such, then Slackware is definitely the way to go - you can get incredibly small installs...unless you want to have lots of X (i.e. graphical) applications, then it doesn't matter which distro you go for, they're all gonna be big.
Obviously there are some distros that are so small they fit onto a floppy, but I'm guessing that you really don't need something that small.
Here's Ibiblio.org's Linux Distro list
- it's fairly comprehensive and gives a rough description of each.