How big a heart attack might lead to Heart Failure?

It's a difficult question to answer. Any area of heart muscle that is not working after a heart attack can potentially weaken the overall structure of the heart. But usually if it's a small area of heart muscle, then the rest of the heart will compensate for long enough without necessarily causing symptoms of heart failure. Of course if it's a large heart attack then it's more likely that the heart will fail to pump as well as it should do. Some of that may well be recoverable in the shorter or medium term after a heart attack.