Snail shells grow with them. Hermit crabs are the only thing I know of where they actually switch shells as they grow.
Snails mutiply pretty rapidly, unless you have some fish that will eat them. I hear clown loches will clear out snails in a hurry, but have never tried this myself.
Personally, I believe snails in a plant tank are a good thing, but some people dont want any. My tanks have pond snails, ramshorn, and malaysian trumpet snails.
Malaysian trumpet snails are particularly useful in a planted tank, as they spend the daylight hours in the substrate. This helps keep the substrate from compacting and forming dead zones.