That is not a WA lens, so don't think about 8x10". It covers about 50deg or so (as compared to a Plasmat, e.g. Symmar, which covers 70deg). The seller who states 24x30cm (that's 10x12" for you non-metric folks
), don't know what he's talking about. He's probably looking at some prints made from negs shot possibly with the lens in question.
Also, the lens covers much less when shot wide-open and the magic of Heliars is shooting them wide-open. At f/22 they are just sharp (i.e. just about as sharp as any other good lens).
//Björn