One of the most obvious differences is size. Standard outdoor hot tubs typically measure 1.8 to 2.5 meters in length and 1.8 to 2.3 meters in width. Outdoor swim spas, on the other hand, are generally much larger. To provide space for swimming or exercise, they typically range from 4 to 7 meters in length and 2.2 to 2.8 meters in width.