From the 2010 KCBS Rules: "Contestants shall provide all needed equipment, supplies
and electricity, except as arranged for in advance. Contestants must adhere to all electrical, fire and other codes. A fire extinguisher shall be near all cooking devices." So there's really no guidance from KCBS about size and type of extinguisher. You should check with the Contest Organizer to see if the local FD has any special requirements.