for the rear sway bar...which one would be better...one would bigger or smaller diameter...?

 

Bigger always leads to more oversteer. I've always been a strong advocate of beefing up both the front and the rear together to maintain the same bias in chassis roll, but I know a lot of people that swear by a large front bar and removing their rear. For me though, understeer is the devil.

 

I went with a bigger front bar but then my original had serious rust issues. However during a Test-N-Tune session I ended up tweaking the shocks so that the Tokico Illumina's were at full stiff and the fronts were at midrange. That got me as close to neutral as I think I could have. But I have a full rollcage and gutted interior so my changes might not work for you.

 

It depends on what your spring rates are, what the tires are (size and type), what your weight balance is, what your alignment is, and what you prefer, oversteer or understeer and how much. There's more, but that's some of the basics. Most with a large front bar and no rear bar will have much stiffer springs, but then if they went to stiffer front springs or softer rears then the rear bar might be needed to regain the balance.