1993-2002 GM F-Body Camaro and Firebird cars came from the factory with 30 mm front and 19 mm rear sway bars.
For the 1982-1992 GM F-Body and most other vehicles the diameter of the sway bars varies depending on the vehicle's model, options and package.
The best thing to do is measure the diameter of your sway bar using a caliper. To get an accurate measurement be sure to measure on a straight portion of the sway bar away from any bends where the bar deforms from bending.
If you do not have a caliper, another crude but fairly accurate way to get a measurement is to tighten an adjustable wrench around the sway bar and then measure between the wrench flats with a tape measure.