Between 1.5 - 3 cm 
The great pond snail  the UKs largest pond snail species and can reach a length of 6 cm although the usual size is closer to 4 cm. Their large size makes them suitable to keep with larger fish. They are excellent scavengers and will eat detritus, fish waste, dead plants, leftover fish food, dead fish, algae and blankett weed. They  also eat hydra and are great for controlling infestations of them. Being a native species the great pond snails can survive the harshest UK winters but can also be kept in coldwater and tropical aquariums. It makes a superior cleaner than most snails found in pet shops like rabbit snails or nerites, and contrary to popular belief in the aquarium hobby ,they rarely eat live plants, only the brown parts, or graze algae from the surface of the leaves. Its also easy to keep their population down if your worried there are to many in the tank or pond as they're quite large and easily removed. Snails make up an essential part of a pond or aquarium ecosystem and would make an excellent addition to a wildlife pond.