Ive been seeing that people are changing out torsion bars and Im not really sure why people change to larger bars. Can someone explain to me why you switch and how to pick the proper bar.
People switch for various reasons. The best reason is that stickier tires are being used that originally came with the car. While cornering the car now has better traction, and the car rolls more. Body roll is disconcerting to drivers (varies with driver) and also can produce less desireable camber. This in turn takes away from the tire's effectiveness.
This must be balanced against road conditions. Making either the anti-sway bar or the t-bars too stiff will reduce the ability of the wheel to smoothly stay in contact with the road. Also even on good, smooth grippy pavement (eg concrete) a stiffer suspension loads the the outer tire more. Eventually it gets overloaded. So there is a limit to the effectiveness of stiffening.
For general street use on typical modern radial tires, a front sway bar is probably the best first move if there is none. Next would be the stiffest factory t-bars, and or something up to 1.03" diameter t-bar depending on front wieght of the car. To the above, add a slightly more aggressive camber and caster. At least the most aggressive side of the factory specs.
On the other hand, if you're driving on sand, or in snow, then stiffer is not better. Nor a is a big bar an advantage for drag racing.