Size of Soundbox and Loudness
Instruments- discuss specific features, luthiers, instrument problems & questions
The general rule of thumb for loudness and 'baritone-ness' is that the larger the volume of the soundbox (not just width or length, but volume), the louder the overall sound... especially when accompanied by more than 4 or 5 square inches of sound hole.
Size (square inches) of sound hole is important to creating volume. There's a complex mathematical solution called the Helmholts Equation (Wikipedia has a good write up on the subject). As a dulcimer gets bigger it needs more soundhole to let out the sound, and it's not a linear progression. John's Uncle Eddie has soundholes that are scaled up from the originals, but it needs maybe almost twice as much area of soundhole to "let it all out". If you are truly interested in maximizing performance, I think you'll want to find someone who can help you solve the Helmholtz equation specifically for the volume of the proposed body.
Mass of the body isn't as important as you might think.
Look at the traditional Tennessee Music Box -- roughly 4" deep, 14" wide and 26'-28" long, with planks (sides/top/bottom/back) averaging about 3/8" thick! Usually on feet to allow the back to vibrate as well, and a solid (seldom hollowed) 2x2 freboard.
Without a lot of "messing about", the simplest solution to a LOUD dulcimer is, IMHO, to build something about the same dimensions as the Tennessee Music Box I described above. Look at pictures of original TMBs to get an idea of how many soundholes/how much area of hole you'll need for something that size. Then here's my suggestion. Make twice as many soundholes as an original TMB of the same dimensions. Then start blocking off holes two at a time, and see the effect on the sound.