"Small" and "horn" are mutually exclusive words, even more so if your objective is a convincing reproduction of piano, and even more so if you want to use low powered amplifiers. "Cheating" with a driver loaded to a TQWT instead of a horn may be your closest option. With such priorities and the small room size, I'll look at something like the Cessaro Chopin, which is smaller than it's successor Wagner. Downside is that, due to being discontinued, it may not be available for audition at a dealearship unless it is a used or ex-demo pair. I have not heard the Chopin, but it is a speaker I plan to hear and probably buy if I like due to it's characteristics and relatively low used price. Ditto for the Horning Eufrodite, it may be a tad big for your space, but due to having 39300391 different revisions, used prices are ridiculously low compared to new.