Blender GE 2.6 FPS Tutorial - FPS Rig Setup P2

I assume you mean the collision box size? Or the different animations? The collision box can be changed using logic bricks, unsure about the animations yet.

but how would you between weapons of different sizes? e.g. rifle to pistol.

Starting to get into the nitty gritty of the FPS rig setup... videos will now be slowing down as I take my time to learn the setup.