Most lap dulcimers have a string length of around 26 - 27 inches. Somewhat shorter than most guitars, I think. As to the load they'll bear, it's considerably less than a guitar. Dulcimers have three or four strings as compared to six for the guitar (unless it's a 12-string), and the string guages are lighter for the dulcimer.
For my dulcimers (and this may be getting somewhat off-topic for this thread), I use banjo strings in guages of 12 for the melody string, 14 for the middle string, and either a wound 22 or 24 for the bass, depending on the instrument. Again, much lighter than the strings I use for acoustic guitars.