I picked up an older 2-3 person hot tub. Spa is the brand. I have not manual for the spa. The motor on the pump says Volts 115, AMPS 16.4/4.4, HZ 60, RPMS 3450/1725, AMPS 40 C. There is a 30 breaker in the wiring box that goes to the thermostat, motor and the on/off switch.
What size wire should I use (8-3 w/grnd ? ) Can I use a 50 Amp SPA/GFCI diconnect breaker.

NO. You need to find the instructions or wiring diagram to see what size circuit they recommend.
DO NOT just wing it. Get the correct information and go from there.

Spas always have a wiring diagram inside the wiring compartment cover.