Usually you would put the fins at the bottom of the body tube and evenly spaced around it. The rocket may be stable even though the fins were a little higher on the tube; it depends on the location of the Cg.
Unless you have friends who own a wind tunnel, flying it is the only test.
yes
A longer fin has increased aerodynamic drag. The geometric relationship between the drag caused by the fins and the center of gravity of the rocket determine the stability of the rocket in flight. Less aerodynamic drag means the rocket can go higher or farther, but it also means the rocket is less stable in flight. This tradeoff has to be considered in the design of the rocket.
Triangle.
The definition of fin on a rocket would be, a thin, protruding piece of metal whose function is to provide aerodynamic stabilization when the rocket is in flight.
fin and tip (cone).
There does not appear to be a "best" size. It depends on a number of factors, to include the bottle used (brand of drink & manufacturer), type of rocket, distance aiming for. One common element to consider is drag. Larger fins have greater surface and mass which will directly affect the altitude your rocket can reach.
It helps stabilise the flight
A fin on a rocket is a flat surface projecting from the body that helps stabilize the rocket's flight by providing aerodynamic forces to keep it on a desired trajectory. Fins increase stability by creating drag and preventing the rocket from spinning or veering off course during ascent.
The size of fins required for stability on a rocket depends on factors like the rocket size, speed, and desired stability. Generally, larger and more streamlined rockets may require smaller fins, while smaller and less streamlined rockets may need larger fins for stability. It's important to consider the rocket's center of gravity and center of pressure when determining the right fin size for stability. Testing and experimentation are often needed to find the optimal fin size for a specific rocket design.
There the most aerodynamic of fin shapes.
With a good design and light tube, it will go 4-5,000 feet.