resolutions expressed as something like "1920x1200" are absolute pixel counts, not dots per inch, and have nothing to do with the physical size of the monitor (except that physically larger monitors can display more pixels and therefore have higher maximum resolutions). a 19" monitor at 1280x1024, for example, simply has 1280 pixels spread across it's physical width and 1024 across its height - not per inch.
If your 19" runs at 1280x1024, then I think you can fit two of them side by side on a 30" monitor like the samsung 305t. It's native resolution is 2560x1600. I have a 24" an a 30" side by side, And if I have a window that is about 2/3 the width(1280) of the 24", I can drag it to the 30" and it occupies half the width. I think the key is that the pixel sizes are the same between the two.
And for what it is worth, I measure the horiontal dimension of the 24" at 22.3", and the 30" at 25.3".