Generally speaking, most mushroom knives come with blades within the 2.5-inch range. This is the optimal size, but there are slight variations out there. The blade is optimal for almost every type of mushroom – you simply do not need more than that.
As for the handle size or the overall size, it solely depends on your personal needs. Those with large hands should get something bigger. Handles vary widely in size – from three to five inches on average. You should know better what size is more suitable for your hands.