There is way more to it as you discover when setting up dirt bikes. The length is irrelevant to the ride height. There are many factors including, wire thickness, distance between the coils etc etc. If the spring is shorter but but carries the same ride height as a longer but softer spring, obviously it will give a much stiffer ride.
Springs on dirt bikes are measured and sold by Kg/mm. That gives the load they will carry, then you need to decide if you need it progressively wound or straight wound. Progressive will give softer initial travel.
Car springs are dumbed right down. But I bet if you actually looked right into it you could actually set your car just as well as a bike. It makes an absolutely incredible difference to how easy a bike is to ride and if you watch a buggy racing something like FINKE, you can really appreciate what is possible.