The required size of the wire depends on the amp draw of the "appliance". Some spas only need 20 amps, while others need 70 amps....and there's everything in between If you use too small a wire, it will overheat, possibly causing a fire.
You don't give ANY information on your spa...brand, model, or what the amp rating listed on it says. Without any of this, all one could recommend is going with what the manual says, AWG6 3.