Trap size is an important part of toilet flushing, because a small trap size can cause the toilet to plug more frequently. A large trap size, usually considered 2 inches in diameter or greater, allows more material to pass through without clogging the toilet.
The toilet trap, or trapway, is the snakelike tube shape on the sides of the toilet. Water and waste flows through this trapway when the toilet is flushed. The size of the flush valve is another important part of a toilet's flushing performance, as larger flush valves allow water to flow into the tank with more speed and force. A toilet with both a flush valve and a trapway that are greater than 2 inches in diameter deliver optimal flushing performance.