Simply hit ESC to bring up the Main Menu and type one of the words below to enable the cheat. If successful, the menu will disappear (non-US versions only) and the cheat will be activated. Type slowly and carefully to ensure correctness.

Cheat Code - Granted Wish

FatSheep - Sheep Grow to 150% Their Normal Size
MiniSheep - Sheep Shrink to 1/2 Their Normal Size
PooSheep - Pastroal Sheep Poo Instead of Graze
CancelCheats - Deactivate Cheats

Note: Only the "PooSheep" cheat will work in the US version of Sheep. The other cheats were added into the game AFTER the US version was complete.