In the main menu, if you move to the right to the Extras section, you can enter cheats.
To enter a cheat, press A and then hit any combination of buttons below with the directional pad on the Wiimote.
Big Head Mode (Story Only) - Down, Up, Left, Right, Right
Dragon Warrior Outfit (Multiplayer Only) - Left, Right, Down, Left, Up
Infinite Chi - Down, Right, Left, Up, Down
Invulnerability - Down, Down, Right, Up, Left Unlock
All Multiplayer Characters - Left, Down, Left, Right Down