That doesn't seem big enough. I have 8 helix aspersas in a tank that's about 30cm x 30cm x 40cm and I'm looking to buy or build one much larger as the tank I have doesn't allow for a lot of exploration.
A large storage bin would probably be a good size for your 16 aspersas.
If you choose to leave them outside, don't try to leash them. You could, however, put bird netting over where they live in the garden to protect them from being eaten by birds. Snails are homing animals. If there's enough of everything they need where they are I don't think they'd wander very far.