The height of the wing (relative to the roof), chord length, width, and profile all matter. Go with a proven wing like the Bimmerworld wing or Brooks wing. APR GTC300 works well too, and is the cheapest. You don't just want downforce, you want to minimize added drag. And all of those wings are good at that.
The angle of attack is adjustable in any decent wing and you can adjust and see how you like it. More AOA is more drag and more downforce, up to the point of stall. I ran a Brooks wing and liked it and Brooks provides great support for club racers.
Make sure your splitter is more or less sealed to the bumper, and 3-4+" out from the front. The limiting factor in aero on production based cars is always the front.