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Pokemon Yellow | GameBoy
First: I'm pretty sure all Snorlax are big.
Second: You must go to lavander town, after obtaining the silph scope, and go to the top floor. There, you will battle a marowak, after you defeat it you will go to the very top floor. Team rocket will be there waiting for you, and when you defeat them keep on going. You will find a man, then, after the talking, go back down the tower. When you're out, go to a house(Sorry, I can't remember which one, I think it's on the left) and you will see the man. Talk to him and he will give you the Pokeflute. Then, go to the Snorlax and press A while facing it. If that doesn't work, try selecting the pokeflute from your bag. The Snorlax will then wake up and battle you. I suggest trying to capture it. Good luck!