Most FPS game dont make the arm bigger in fact they used something called fake arm here is a post showing how to make a fake arm and adding gun to it which will looks like these FPS game you’ve played The First Person Element Of A First Person Shooter
but if you dont need fake arm and want to make the actual arm bigger then whenever any player character loads in they load something called Humanoid description HumanoidDescription | Roblox Creator Documentation and It has some children class(a string value i think) in it which you can use to scale character limb such as Left Arm or Right Arm