>>528131852
If you’re referring to what a snail body could physically accommodate during mating (again, purely biologically), the organ of the mating partner would usually be no larger than half the length of the snail’s body and relatively slim—proportionate to the snail’s own anatomy. Snails’ bodies are soft, flexible, and capable of accommodating mating structures of that relative size without harm.
If you’re imagining this hybrid in a natural, biological context (not in a human sense), it would follow the same principle: mating structures proportional to the character’s snail shell/body size would be biologically plausible. There’s no realistic scenario where something disproportionately large (relative to its own body size) would be appropriate or safe.
