How to choose correct drill bit size for screw and wallplug?
The wall plug may have a size molded into it, or the packaging will say. At any rate, the right size is usually the diameter at the widest point of the plug's taper (if present), not counting a flange. It should not take a lot of force to set the bare plug.

In the case of masonry (and most other materials) you can err on the small side and work larger until you get a good result.