They're represented as almost-fully-grown (if not fully grown) high schoolers. They shouldn't be waist-high to a man.
They're near his shoulders, and he's been drawn as a very tall man. Much taller than his female counterpart, and Suzuya is shoulder height to him. The perspective is weird in that second last panel as it's from above everyone.
A few pages back, you can see both of them clinging to his side, and even if they're bent over a little, you can tell that even standing stock straight they'd only reach his shoulders or so.