What's an avatar's base physics shape?

An avatar's apparent shape can vary quite a bit, depending on settings and attachments, but what's an avatar's basic shape that's used to determine physics?

I'm very curious because after working on vehicles for a while, it seems that avatars have the perfect bottom shape for a vehicle

So does anyone know? Is this an observer error?

the appropriate LSL function call will return an avatars size as an elongated(tall) cube. a bounding box. also you can stand on their heads, so i'm gonna go with that.