I had a 15'x3' EasySet and the 4'x10' was too much. One day, the water reached about 107deg! It was the biggest hot tub you ever saw! Usually it kept the water in the mid to high 90's--here in North Central NJ. Unless you are in northern Canada or a place where you get about 2 hours of sun a day (think Seattle, WA, or Binghamton, NY), the 8' is probably best, and the 12' is serious overkill--you could par-boil with that!