Think of the distance between the front wing and the back wing like the distance between trucks on a skateboard. The further apart they are the longer the turning radius becomes. The shorter they get, the quicker the turns become. The fuselage lengths work the same way. Longer for longer radius turns plus added stability. Shorter for quicker turns but the foil becomes more pitch sensitive.
Carbon weighs less and adds to the overall lightness of the foil set.
Aluminum is strong, durable and also lightweight compared to other alloy fuselages on the market