>>5509881What material are you putting the holes in?
The pegs on a 6mm Revoltech joint are ~3mm in diameter, so the following info is based on my experience with only the 6mm joints.
For flexible materials like rubbery plastics you can use a 7/64" drill bit. Stiffer materials with very little give (such as hard plastics) will require a #32 gauge drill bit.
Pic related for those who need a quick reference of imperial to metric drill sizes.