So there's a number of concerns that you want to think about. Some of these you'll have to determine experimentally, others you'll be able to calculate and then iteratively improve based upon your observations.
It should be understood / obvious that the higher up you can get the car, then the faster it will be able to go, and ultimately travel the furthest distance. The challenge though is transferring the vertical potential energy into horizontal motion. So the taller and longer of a ramp you can make, then the furthest your car will end up traveling.
Use a fixed, rigid plate to form the initial ramp. Run some experiments to determine the steepest angle the plate can form but still allow the car to roll smoothly off of the ramp. Having the car crash at the end of the ramp doesn't do you any good and wastes any energy that was converted.
Once you know your angle of attack for the ramp, you'll want to start experimenting with various ramp designs. This is where the principles of beam deflection start coming into play.
I'd look into C and I (or H) beams as they ought to provide optimal stiffness at the edges of the beams while minimizing the amount of material required. An H beam design may allow you to join two beams together and fasten them in areas where the car won't be impacted by the staples. Another advantage of those beams is that they'll provide rails to keep the car on the track. That said, you'll need to fold under the edges of the H beam where the ramp meets the horizontal surface the car will travel along.
Likewise, look into triangular beams to create the struts that will support and lift your ramp up into the air.
If you'd like to do some of the beam calculations ahead of time, it's pretty easy to search for Young's modulus of elasticity for paper. That parameter can be affected by paper weight and coating, so that's research you'll want to do on your own.
Knowing the angle of attack and your preferred beam design will allow you to lay things out on the sheet of A2 paper that you have. This represents your final constraint inasmuch as you have a fixed amount of material you can use.