Stirring ensures that all areas of the pile receive necessary oxygen, distributes heat, and also prevents bad odors. A pile that is decomposing properly should generate temperatures of 140 -160 F at its center. This heat will kill most weeds, plant diseases, and insect eggs. Turning the pile distributes the heat through out all the compost. When the compost is finished, the pile will no longer heat up.
Large piles generate more heat than a small pile.
If your pile is not heating up properly, don't worry. A cool compost pile will still generate good compost, but it may take a 3 - 6 months. A hot pile can generate finished compost in 1 - 3 months. The smaller the pieces are when they are added to your compost pile, the quicker they will decompose. Some materials will only decompose effectively if added in small amounts to the compost pile.