Assuming a standard size guitar with around a 25" VSL, the bass "E" string is anywhere from 0.046 to 0.056. The Blue Lion bass dulcimer I play in our dulcimer group has a 27.5 inch VSL. The string gauges are 0,024, 0.028, 0.032, 0.052. All of them are bronze wound. In a set of medium guitar strings, the 4th string (D) is usually around 0.035. If you use that as your "melody" string, you can size the middle and bass appropriately.
Hope this helps.
"The dulcimer sings a sweet song."