You could try Zipslack since that's fairly complete and is still small. The biggest problem you are likely to face is that, as far as I am aware, there are no BIOS manufacturers that support booting from USB devices, so you'd still need to either have a boot floppy, or use loadlin.
I think this has been discussed before now on these forums, but I'm not sure... have a peek.
Oops... where are my manners? Welcome to LQ.