6mm is the standard diameter generally all airsoft guns are designed to shoot. They can come in different densities or weights, which would affect the performance of the gun based on its FPS, but they will always come in 6mm. Airsoft BBs are generally available in plastic, but bio-degradable options are available as well. This makes them different and often incompatible with pellet or general BB guns that are made in different sizes and materials not compatible with airsoft. As mentioned the FPS of an airsoft gun is affected by the weight of the BBs as lowered power guns would have trouble shooting heavier BBs while high powered guns could end up destroying BBs in lighter weights causing jams and damages to the gun itself. Balancing these issues will increase the performance of an airsoft gun and its longevity as well.