Probably the most frequently asked question I get from my web site readers has to do with what strings I use on my dulcimers. I always tell people right from the start that I'm only giving my personal preferences, and that they should not take these to be any sort of standard or default. String gauge preference is a very subjective thing.
Most of the basic bare-bones dulcimers I'm working on now have either 25" or 26" VSLs (Vibrating String Length - nut to bridge). Since I'm setting up the instruments to be tuned DAD, I choose gauges that work pretty well for that specific tuning, although tunings that are close - like DGD, or DAC - should work fine too. If I want to spend a bunch of time playing in DAA, I will use a heavier, thicker gauge for the melody string.
Here are my preferences:
I usually prefer a nickel-wrap string for the bass. This is purely preference, and many other players and builders like phosphor bronze or some bronze/brass combination.
Keep in mind that the dulcimer tends to be a pretty delicate, gentle kind of instrument (my dulcimers are not known for their volume), and the string tension on a mountain dulcimer is much less than it is on a guitar, for instance. Now, if you want more volume and punch, feel free to try thicker gauges, which will make your strings feel tighter, and they will be a little harder to press down to the frets -- making it seem like you have a slightly higher string action. Experimenting with string gauge is one of the healthiest activities I can think of -- be brave!