best to buy a zoomie kit, dont get the double wall, you dont need the extra tube, adds to the cost anyway. The kit is long tubes with the bends on the ends already in, just cut to length and weld to flange.Depending on your car, you may need to snake some around steering .
The kits are 2 1/2", may be too big for your car.
You can also use strait tube and get 4 180 bends , cut them in half and you can do 2 cylinders.
Lengthe doesnt matter much, what ever fits the car. Diameter is improtant, as big as you can make them is good.
Best to make some kind of support on the bottome of the zoomie, to the car frame. This helps to prevent cracks at the flange.