to choose based on your position, age, and style of play. Shin guards should cover most of the area from your ankle to your knee. Measure your shin from just below where the knee bends to about 1 inch above your shoe top. This is the proper length of your shin guard. Some manufacturers label their shin guard sizes according to age.
an airsoft gun can break skin at 300 fps it all depends on what weight bbs you use like .2, .25, .3 ect. it also depends on how far you are from the person when you shoot them. it all depends. i was hit from a 300 fps gun and the bb popped my skin open and ive been hit by a 590 fps gun and it didnt leave a mark. im also on an airsoft team with people that have alot of experience.
Depending on the mechanism for propelling the pellet, an airsoft gun can be operated manually with a spring-loaded air pump, or on an automatic cycling basis which is implemented either pneumatically with prefilled bottled gas (such as compressed green gas or CO2), or mechanically driven by an electric motor gearbox.