How bad would a rattlesnake bite affect me? I'm 6'9'' tall and weigh 300 lbs.?

It depends entirely on how bad the bite is, and how close it is to a major artery (which will affect how quickly the venom is spread). An adult rattlesnake would at the very least, seriously mess you up, and quite possibly kill you depending on how much venom the snake injects upon striking. Assuming you don't die, you could probably expect permanent damage to whatever limb the snake bit. If you are really unlucky, they may have to amputate the limb because it has died. With a baby rattlesnake, however, you are dead. The venom is much more concentrated at that stage of life, and although the amount entering your body will be less, it will be much more concentrated. And you will die.