A good sized tank is a 20 liters (5 gallons). I would suggest having one garden snail per 4 liters (1 gallon).
For the substrate i have used bird grit as it will give the snails calcium. This can be found at your local bulk food store or pet shop. If you choose another substrate it would be a good idea to provide another source of calcium such as a cuttle fish bone or crushed eggshells.
other substrates you can use: Bark Aquarium gravel Peat moss And soil