So how do you choose which torsion bar size to install?

It would be a good idea to take care of a few other things before choosing your torsion bars.

1. Select the wheels and tires that you want to run.
Wheel size can limit your brake selection.
Tire sidewall profile affects handling.

2. Do any brake upgrades. Being able to stop is important.
11.75" rotors should fit in 15" wheels, or if you have bigger wheels, you can get bigger brakes.

3. If you want to remove any weight from the car, or shift any weight from the front to the back, now is the time.
Try to have 50% to 55% of the total car weight being supported by the front tires. You can check your work on a weigh scale at the local truck stop, dump, or recycling center. Get a weight with only the front wheels on the scale, and get another weight with the whole car on the scale.
(Front weight / total weight) x 100 = front weight %


Ok, now that the cars weight has been determined, you can select torsion bars.
If you're not sure what to choose, AR Engineering suggests a rule of thumb that torsion bar wheel rate should be 10% of your front end weight.
The torsion bars job is to support the weight of the car, so it makes sense that the cars weight should be considered when selecting torsion bars. Heavier cars need higher spring rates. Every car is different.

http://arengineering.com/tech/torsion-bar-tango/