Increase in string length means an increase in string tension. Longer string lengths tend to sound twangy and nasal, and are especially appropriate for the traditional dronal style of playing. However, reaching chords along the fingerboard is easier. If your hands are small, and you plan on playing chords on the dulcimer, consider one with a shorter string length. Frets Frets are specially shaped lengths of wire seated in the fingerboard at mathematically dtermined intervals from one another.
Frets mark the locations of notes along the musical scale.
The soundbox dimension most affecting low-end is the depth of the sides. The shape of the sound box, hourglass, teardrop, rectangle, ellipse, and so on, doesn't affect the sound of the dulcimer very much, although there is no general agreement on any of these parameters. Unless a soundhole design removes a majority of the vibrating soundboard, its shape has little audible effect. The dulcimer is played on one's lap or on a table, with its tuners to one's left and strumming hollow on the right. All you really need are your hands, ears, and a good instrument.