I simply do not understand the science of drilling holes in walls and then getting the right-sized plug to fit. This morning I drilled the first hole for a towel rail, and fitted the plug - no problem. Then I drilled the second hole - same drill bit for the same sized plug. However, when I came to insert the plug, would it go in? No it would not. Why does this happen? How do you match the drill bit, plug and screw? The usual answer is that you look for the numbers given on the plugs/screws/bits. But what if these numbers are not present? How do you do it by eye, which would be more convenient anyway.
There at at least two different ways of specifying masonry drill sizes. In the old days, you would buy a No. 8 or No. 10 bit (say). This would drill the right sized hole for a fibre rawlplug for the same number of screw size. Inn those days, you needed exactly the right rawlplug for each size of screw.
But that has all changed withe plastic plugs - which are usually designed to take a range of screw sizes. The plugs usually come on a plastic strip - and you tear off each one as you use it. On the back of the strip, it usually specifies the screw sizes for which the plugs are suitable *and* the drill size to use. This may be a No. X drill, but is more likely nowadays to be specified in mm - e.g. 6mm.
To come to your first question, it depends to some extent on the material which you are drilling. In soft breeze block - or if you hit a mortar joint - a plastic plug will usually go into a slightly smaller hole, because it will enlarge the hole slightly as you tap it in. In brick - particularly very hard brick - there is no give, and the plug will buckle if you try to hammer it into too small a hole.
In hard materials, I drill the hole size specified on the plug strip. In soft materials, I tend to use one size smaller in order to get a firm fix. Try to use the smallest hole you can get away with. [As a rough guide, if no drill size is specified, measure the diameter of the plug (just below the flange) with a vernier caliper and use a drill size 1mm smaller. Alternatively, hold the plug against the working end of a few masonry bits, and select a bit which is just slightly smaller than the plug.]
Assuming you're using size 8 screws, you really need masonry bits on hand of
5mm/5.5mm/6mm and 3 sizes of plug. In soft stuff the 5mm bit may bash a 6mm hole and the red plastic plug will fit perfectly. In hard stuff the 6mm drill hole will be precisely 6mm and the red plug will also fit perfectly. Usually it's somewhere between the 2, which is where the 5.5mm comes in. If the red plug is a sloppy fit, tap the small (usually yellow) plug inside the red. If that's too tight, yellow in brown is worth a try. Personally, I miss those cut to length plastic plugs that you could hammer into tight holes. Ideal for twinthread screws but not sighted in the sheds for many a long year. Anyone?
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