screws
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?
I usually just poke them into the wall (unless they are the screw in kind obviously, or the kind with a large collar). Put a screw driver on them and give it a sharp whack, then patch the hole. I find it's easier and faster to patch the slightly larger hole, than it is to try and dig them out and then patch the hole anyway.
For the larger collar ones, try to bend/mutilate the collar enough then poke them through the wall.
Be careful when you do this, you don't want to put the screw driver and/or your hand through the wall as well.
In a brick wall you could drill them out. Find a bit that is about the same size as the hole in the anchor, then carefully drill it out.
If you have an identical screw, you can probably take it to Home Depot or Lowes and use the thread gauge mounted in the hardware section. (The HD in my area has one, but I'm not sure if all of them do.)
This website also has several printable size gauges for screws, bolts, washers, and nuts.
Otherwise, you can buy one. They're usually labelled as "Screw ID Gauge" or "Thread Gauge." Home Depot has them in the hardware aisle for $2 or $3.
Best Answer
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.