Can I use 14/3 wire 20 amp double pole breaker for a 240 volt baseboard heater of any watts? What's the electrical code?
Only if the heater draws less than 12 amps would 14 AWG be adequate. And that would require a 15 amp breaker, not 20, to properly protect 14 gauge wire.
Space heating devices require wiring to be 80% derated which is assumed by the electrical code to be "on" 100% of the time.
A heater rated up to 24 amps requires 10 AWG and a 30 amp breaker.
A heater rated up to 16 amps needs at least 12 AWG and a 20 amp breaker
A heater up to 12 amps needs at least 14 AWG and a 15 amp breaker.
The heater will have volts, amps, and watts on its "ratings plate".