What do you mean by the smallest? Are you trying to see how small of a setup you can install or other similar test, or do you just want something that fits easily? You can easily install any distribution you want, just dont install all of the extra packages you dont need. I dont know about other distributions, but I know that Mandrake makes it very easy for you to select (or avoid) extra packages, and shows you exactly how much space your install will take up. You can also use Mandrake Move if you want a version to run off of a CD.
I would try to avoid running a less-common distribution, or a distribution specifically chosen because of it's size or any other single quality. Dont restrict yourself based on anything. Find a distro that you like, and install it. You shouldnt have any trouble making it fit, regardless of what distro you use.