The smallest iso size? The smallest memory footprint? With GUI? Without GUI?
TinyCore is the smallest GUI based modern system I'm aware of at ~10 MB or it's MicroCore console only alternative at 6 MB. Slitaz is something like 25MB, I think. DSL is dead, but comes it at around 50 MB. There also TTY Linux
, again no GUI, which is 13.5 MB, and can run with as little as 16 MB RAM (vs MicroCore which needs at least 24 MB from what I've heard).