But as I remember it, the thing with Harry being a Horcrux was entirely unintentional, and Voldemort doesn't even know about it. That means there's also no way for Dumbledore to know it - his theory about Voldemort *wanting* to split his soul in seven is correct, neither he nor Voldemort knows that it's inadvertently been split in eight instead.

As for other things you said: I would guess intent plays a role - a bit like the Unforgiveable Curses, maybe. Killing in self-defense or by accident wouldn't split your soul, only intentional, premeditated murder. Still, it does seem likely there have been Dark wizards in the past who committed such murder many times, they just didn't make more than one Horcrux. Dumbledore says himself that Voldemort only made Horcruxes for particularly significant murders - which clearly means he's committed more than six (or seven) murders.

And I don't think there's any "size" aspect to it, no - the "split in two" is perhaps a confusing way of phrasing it. Tearing a piece off would probably be a better phrasing.