Impressiveness is the quality of being impressive — the wow that a thing has, turned into a noun. We talk about the impressiveness of a view, a building, a performance, a score. It is a fairly formal noun; in everyday talk people more often say “how impressive it is”, or they name the thing: the size, the skill. You will see impressiveness in reviews and careful descriptions. It is not a common spoken word. Though actually, impressiveness can be a little cold: it names the effect, not the feeling. “The impressiveness of the ceremony” tells you it was grand; it does not tell you if anyone was moved. Size, skill, and importance can all give impressiveness, so the word stays general — you may need a second sentence to say which kind.