It depends on the guides you use. If you have gone are running Carrera cars with stock guides or some of the aftermarket wood guides, they will hang up a lot easier than most other stock slot cars due to the length and thickness. First I will define my terminology as it is often used in different terms in discussions among slot car folks on the boards I read. Mathematically, radius refers to the distance to the center of a circle or 1/2 of the diameter. I believe your CNC program will read radius as that.
I have an inside slot on my track that is 3.5" radius (7" inch diameter) but that is very tight and will cause hangups for large guides, especially Carrera as they are thick as well as long. To drive it you have to kind of toss the car in and let the rear end swing around to point the car out the exit. It is definitely not a curve for newbies. If you use this type of curve, it is best to use them in opposite pairs so that the disadvantage of a slow tight inside lane is apportioned to both border lanes equally.
I personally like this pair of sharp hairpins but they do take some getting use to and definitely trimming and grinding of stock Carrera guides. For general purpose, I would recommend nothing less than 5"radius (10"diameter) on the inside slot if you have newbies racing or you don't want to fuss with guide modifications. A minimum of 6" radius (12"dia.) would probably be best for most people's taste.