anything over 340 (870 bars) you're going to want good shocks.
anything .990 and up you're going to want really good shocks.
t-bar size selection is based on a bunch of parameters but it basically comes down to: how you want your car to handle, what level of harshness in ride you find acceptable, and how much you're willing to spend (bigger bars are more $$ and require more $$ shocks to tame them).
snapping giant bars in a car isn't going to magically make it handle like it's on rails.
