The word hereof is actually not a legal term. Its meaning in court documents is no different from its ordinary dictionary definition -- Merriam-Webester defines hereof as meaning of this.. It's one of those words used by lawyers and nobody else.
The variety of cabbage commonly known as the Flat Dutch, shown in Fig. 6, of which there are many strains, is a moderately long-season variety. The head is very solid and flat; it measures about 12 inches across and 8 inches deep, and weighs from 8 to 12 pounds.
Edit: I did find something on about.com suggesting that a medium cabbage is two pounds. Unfortunately it claims that a two-pound head of cabbage makes 10 cups shredded, but that a medium head makes four cups. I think the ten cups is way, way closer to correct, based on experience. If you do want a volume, that would probably be a good estimate.