Frank, the inside diameter will depend on your shocks. On our team, we use the 2.5" I.D. with Penske and JRZ shocks. The spring length is 8". We use helper springs to keep them engaged with the shock collars during full droop.
The optimal spring rates will vary with many things such as driver preference, chassis set-up, and your shock valving just to name a few. When you have your shocks valved, they will want to know your spring rates to do a proper job.
If you are coming from a stock set up, you might like 400 front and 700 rear as a starting point. Many of the advanced drivers in the VRL are using much stiffer spring rates. Alot will have to do with how hard you drive your car, how much aerodynamic downforce you have, how much your car weighs, how much body roll you are willing to tolerate, the type of tires you use, etc etc.
It's not a bad idea to keep a spring inventory in the range of where you think you will start and end up, say in 50 pound increments. I've actually changed springs as much as three different times at one track event to find the ultimate set up for me.
I think the more you get comfortable with your car, the stiffer you will go until you can't feel the car losing grip before it's too late. Some guys are at over 1000 pounds in both front and rear. Personally, I like to go as soft as possible without having uncomfortable body roll during cornering.