The size will also depend upon trim (verts often have larger bars, etc). The last one I looked up was bushings for the 94 GT and there were 3 common sizes used for GT hardtops alone.
If you dont have a Vernier caliper, one can measure circumference or get diameter by closing a crescent wrench tight around the bar and then using a tape measure to note the distance across the crescent wrench. Crude, but it gives an idea.
Good luck.