I'm guessing much, much smaller than that, taking microsnails into account.

https://www.atlasobscura.com/articles/found-the-worlds-tinie...

But let me just add up their lengths, along with the lengths of all giant african land snails and all other snails, and divide by the total number of snails in the world ... is there a statistical trick that could answer this, like the one for counting undiscovered species? Probably not.

Perhaps they meant slugs. In German at least, both are called Schnecken (just one is naked), so perhaps that persists in other languages as well.

I think it's more likely different species in different places. I, in NZ, don't think I've ever seen a slug as much as 5cm long, let alone 9cm.

Look up banana slug. Common where I live.