In fire red how do you move the big Pokemon in route 12?

To move the Snorlax blocking the path on Route 12 in Pokémon FireRed, you will need to obtain the Poké Flute. Here are the steps on how to get the Poké Flute and move the Snorlax:

1. In Vermilion City, go to the house located to the right of the Pokémon Fan Club.

2. Talk to the old woman inside the house and she will give you the Poké Flute.

3. Exit Vermilion City and head to Route 12.

4. Use the Poké Flute on Snorlax by selecting it from your Key Items in your bag and using it.

5. Snorlax will wake up and move out of the way, clearing the path.