I saw some 23.1 inch 1600x1200 monitors listed at prad.de, but most of them were from 2006 or older, so they might be difficult to find. A 4:3 resolution on a 30 inch 16:10 monitor has an effective diagonal of about 26 inches (I measured on mine). If that's not big enough, ssddx's suggestion of two monitors in portrait mode could give you an even bigger diagonal measurement, provided you can deal with the bezels in the middle.