I dislike drilling holes, as it irreversibly damages the wall. Therefore I am very careful not to make too much of them. And in the cases I think it is unavoidable, I like to make as tiny a hole as possible. However, I'm not sure which minimal diameter is needed to withstand the force the screw applies on the plug an the wall.
How can I know the optimal size of the plug for a particular purpose? Which factors do I need to take into account?
You need to make the hole the "right" size - neither too big nor too small.
You should choose the size (diameter and length) of screw based on the job you are doing. You should then choose the size of wall plug based on this.
The internal diameter of the wall plug is determined by the size(s) of screw it is designed to take. The external diameter of the wall plus is determined, in part, by the load it's designed to take. A "heavy duty" plug will be bigger than a "light duty" one.
You should then choose the drill bit required to fit the plug snugly. This should be printed somewhere on the packaging. You might find you need to go one drill size larger if you are using the maximum size (diameter) screw recommended for the plug.
