Change my 220v 50 amp breaker to 70 amp on existing wiring to pool panel
All I am trying to do is answer your basic question. Simplistically put, the NEC requires electrical conductors to be protected by overcurrent protection that matches the current carrying capacity of said conductor. So in your case you must run a #4 for your 70A breaker. 

If you are trying to determine a wire size you should be looking for a marking similar to this.



The reason everyone is asking questions is because we think we could help if we had more information. See comments above.

PS - Instead of a 12 your looking for a #8 or 6 or 4.