Honestly, since it's all fantasy and all that, I'm just gonna say as much as he needs to stretch his mouth, though I've never thought about how much he could fit. I can imagine Leaf could fit an entire person in his mouth, two if he's swallowing them both together, maybe he could fit two whole people inside. Or if it's one of those silly size things where they're small when they enter my mouth, who knows! If Leaf's got a massively long tongue, then it probably has to be pretty big to store that...