If you are just shooting flat art, then the size of the Light source will not matter. What is most important is to have even illumination over the entire work of art. This is usually best done with 2 Light sources at 45 degree angles to the flat art and a good distance away from the art, so that the meter reading at any point on the artwork is the same. Then be sure your camera is color balanced to the light. Any of the two light kits for either the StarFlash (Strobe lighting System) or Starlite (continuous lighting system,) will work fine. You may want to consider what other photography you will be doing, as SoftBoxes tend to be preferred for close-up portraiture & product photography.
If your art has any 3-dimentional quality, like an oil painting, then you would benefit from larger SoftBoxes, though you would still want to move them a good distance away from the art, so the meter reading is the same at any point in the image area.