This is a very useful size comparison. Thank you, Horikou-san. On a slightly less joking note, I've never tasted black bread before. Is it that bad?
If you wanted something to compare, imagine chewing on something like a hybrid of sponge and paper; that's the texture. For the aroma, imagine something like a mix of burnt flour and plain oatmeal. For the taste, it doesn't really taste like white bread; the closest would be naan.
Of course, that's black bread from a german baker that's been adjusted to fit general population's palate. The original, I'd presume would be much harder and drier.