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Hermit crab shells sold online are typically sold based on the size of the opening. When purchasing new shells for your hermit crab, measure their existing shell and then choose several shells that are somewhat larger. Hermit crabs typically prefer a shell that fits snugly allowing them to fully seal off the opening to protect themselves from predators and dessication.
The photo shows how to take measurements properly.
Ecuadorian (C. compressus) typically prefer a shell with a D shaped opening and commonly wear shells that are too small.
Other species will wear a variety of shells, both with circular and D shaped openings.