I would use a 5/8 drill bit which is almost exactly 16mm.
Most metals are initially drilled with a narrower drill, forming a pilot hole. Then the right size drill is used to complete the drilling of the hole by widening the pilot hole.
I drill a pilot hole the size of the 'barrel' of the screw. -I always use a combination drill bit for sinking screwheads. The first part of the bit is the diameter of the 'barrel' of the screw and the step, one inch higher is the size of that screw's head. (The barrel is the diameter inside threads) -I have a set of these drill bits for #6, #8, and #10 screws and they are great.
size 6 rings start at 10 then just add the size to it,lets say its a size 6 so,10+6=16mm
That depends on the size of the finish hole. It should be big enough so that the larger bit will grab in the hole and not skate off.
16mm
16mm
Drill bits are not universal. You choose the size of the drill bit according to the size of hole you like to make.
Depending on the material, I would work with a 3/8 or 11/32 bit for a pilot hole.( If the material is quite soft (aluminium for instance) then I go smaller with the pilot.)
Roughly 22 x 16mm, 3g
4mm x 16mm ST
For a cut tap the correct drill size is a #17 drill, which has a diameter of .173 inches.