They can get well into the thousands, so I would say 2-3 20mm test tubes. Their queens also breed in the nest, so theoretically they don't have a size limit except how much you feed them.
It probably depends in the species and population, as here in CA we don't have true S. molesta, just similar undescribed species. That being said, true S. molesta are probably more entertaining to keep as their colonies probably get larger than most species around here and also mate intranidally, which is pretty cool. Reminds me of Monomorium ergatogyna, which are another inbreeding species I've kept before.