If the smallest FOOM seed were very small (like a few hundred bytes) then we would expect evolution to have already bumped into it at some point.

Evolution: 24 myths and misconceptions -> Evolution myths: Evolution is limitlessly creative

[...] some features cannot evolve because a half-way stage really would be of no use. For example, two-way radio might be useful for many different animals, for making silent alarm calls or locating other members of your species. So why hasn't it evolved? The recent invention of nanoscale radio receivers suggests it is not physically impossible.

The answer might be that half a radio really is useless. Detecting natural radio waves - from lightning, for instance - would not tell animals anything useful about their environment. That means there will be no selection for mutations that allow organisms to detect radio waves. Conversely, without any means of detecting radio waves, emitting them would serve no useful purpose.

A few hundred bytes of code might be enough if you have a suitable substrate but the substrate itself has a certain kolmogorov complexity. Evolution does not differentiate between software and hardware.