Best Answer - Chosen by Voters
Okay, here we go. Go to a spa dealer or manufacturer and ask to sit in an EMPTY tub. One size does NOT fit all. If you are 5 ft. 6 you need a different tub than if you are 6 ft. 5. Try a number of them until you find one that feels right. You will be more buoyant when it's full of water, but you don't want to have to hold on to keep from floating away. Next, discuss with the dealer exactly which parts you want massaged most. Upper back, lower back, neck, shoulders, calves, feet, etc. There are a gazillion different types of jets (and jet clusters) each of which is designed to massage certain areas. (Remember, a spa is MORE than a soak in hot water. If that's what you want just fill the bath tub and soak away!) A spa is more like a marriage counselor with bubbles.
Next, try to avoid the temptation to over buy. Seems everybody wants a huge spa. Truth is, you will generally be in it either alone or with one other person. Why pay for way more spa than you actually need? My first one was a big one. Several spas later I have a tiny one that seats 4 people tops. Usually I am in it alone, or with my bride. Now about horsepower. The dealer can direct you best. Depends on the type and number of jets used. Soaking in a spa should be relaxing. You shouldn't get bruised from sitting in front of a too powerful jet. Flow rates are very important when properly sizing a spa pump (or pumps.)A good dealer will know best. If he doesn't, then walk away and find one that does. All of them are going to tell you their brand is best. Few will actually try to "customize" a tub just for you. That's the one you want to use. Different sizes, seating arrangements, jets, and then colors. Then the equipment. All of these factors make a spa a pleasure, or a disappointment.
Hope this helps.