First off, what size sway bars come stock on a 1990 Iroc-Z. I don't have calipers to measure them myself and I can't find any info on the net. I need to replace the squeaky bushings.

What exactly does an adjustable panhard bar adjust?

... an adjustable panhard bar will adjust the rear end to center when you lower the car.

As far as measuring the sway bar, use a cresent wrench. Just clamp it on the bar, around the center somewhere, and slid it off and measure between the jaws.

Thanks.

I used a Craftsman Professional crescent wrench on em since it has jaw size printed on it and it turned out that I have 36mm front and 24mm rear. (For anyone else looking for the same info.)

just measure the circumfrence with a tape or string.
then

C=2(pi)r => r=c/(2*3.14159265359)
Diameter=2r => D=2c/(2*3.14159265359)

D=c/pi easy as that

I use a set of cheap plastic calipers my dad has had laying around his workshop for years. I dont know where he got them but they couldnt have cost more then 10 bux. Theyre accurate enough for checking sway bars and checking bore/stroke on an engine to tell if its what its suppsoed to be or not. The first time I measured my bars I tried to use the string method but turns out I was off, I calculated it as a 32mm and its really a 30mm.