perhaps bolt may have been a better word than screw, if you want to something that connects with a nut and washers. Just get the biggest size that will fit through the hole in your caster. I guess a screw would work but I think for casters, taking the weight of the cabinet, a bolt would be better. Larger than #8 though. This type of part, doesn't really matter exactly what you use, as long as it ends up being sturdy.