that sounds like a fun experiment! But unfortunately you are right, the size difference is shaped by genetics as well as an environmental condition that might be hard to pin down.
For the genetics, It would indeed take millions of years (or at least 100,000s) for the genetic adaptation to kick in to create that change.
As for the environmental conditions, another source of gigantism is being on an island. As the small number of species expand to fill out all the niches in the tank, they may get larger simply because the environment allows it with no predators.
The tank would have to be enormous to get those niches I think. This paper describes an archipelago of islands and shows that you tend to get smaller animals more easily with smaller islands, and larger species need larger islands. Large here is millions of square meters.
Both of these processes take many generations. This is not my field but this paper studied 200 generations of plants and showed only migration and lifespan changes. Size adaptations that come with changing ecological roles might have to take 1000s of generations. We'd have to freeze ourselves to see the giant slugs. I'm sure there's a better answer out there, but this is what i found so far.