If you already don't know by other post of air guns, or penumic launchers, a air gun consists of a pvc air tank and a valve that allows the tank to flood the air in to the barrel launching the projectile.
Having a larger tank allows you to have a more powerful round while having a larger, and thinner barrel allows a more accurate shot, and a larger barrel, allowing more of a variety of things to launch.
having a large tank adds a lot of weight to the gun and will slow you down, a thinner berral is more likely to break under the pressure of the burst, and a larger barrel could not fire the round if to larg
and a longer barrel will definitely add coast to your maneuverability
For my gun i picked a extra large tank, a medium with barrel and a vary long barrel, my gun is vary power full and fairly accurate.