If you have googled spa tubs, you know they come in a myriad of sizes and styles. First, you need to determine your goals for your spa tub before you figure out the ideal spa tub measurements.
Will your spa tub be used for only personal use or do you plan on using your spa tub for your neighbors to join you every night? Do you want your time in your spa tub to a time of quiet or a social gathering? Spa tubs range in size for two people, up to eight. Determining how you will mostly use it, determines the size you want. Two to four-person hot tubs are as small as 5 ft. x 5 ft., where an eight-person hot tub is as large as 9 ft. x 9 ft.
Another consideration is the amount of money you are looking to spend on your spa tub. When it comes to spa tubs "you get what you pay for.” After you establish how many people you want your spa tub to hold, other considerations are if you would like a lounger in your spa tub, how many jets you would like and if you want to add all the bells and whistles, like Bluetooth, or a tv. Maybe going smaller with the tub and higher on the extras is what you want -- or vice versa.
Before deciding on what the ideal spa tub measurement is for you, think about the extras. The bigger the spa tub = more water = more energy in keeping the spa tub temperature hot = more continual cost. Another energy factor is how energy efficient your spa tub is and how often you need to run it. Lastly, just like pools, you need to add chemicals to your spa tub to keep the pH levels where they need to be.