At the title screen enter the codes for these effects:
Big Heads:
Down, Right, Up, Left, A, B, Y, X, White, Black, Start + Back
Flat players:
Press Left Stick twice then press the Right Stick twice then press L, R, Black, White, B, A, Back + Start
Unlock Europe Stars:
Win International Cup with England, Spain, Portugal, Holland, France, Germany, and Italy
Unlock Africa Stars:
Win International Cup with Cameroon, Nigeria, and South Africa
Unlock Asia Stars:
Win International Cup with Japan, South Korea, Australia, and Iran
Unlock America Stars:
Win International Cup with Argentina, Brazil, Paraguay, Mexico, and Chile.
