You decide whether to pay as the enters-the-battlefield ability resolves. If you do, you immediately create the Giant Wizard creature token and attach Giant's Amulet to it. No players may respond to your decision to pay or not, and no player may take actions during this process.

The Giant Wizard creature token enters the battlefield as a 4/4 creature. Any abilities that trigger when a creature with a certain toughness enters the battlefield will see the token enter as a 4/4 creature.