Sometimes you may find this size stamped on a tap itself, or you may have to look up the mating drill size on either a chart or somewhere else. Chances are however, that if you bought a matched set of drills and taps then everything is already set up for you in the package. It helps enormously to predrill a smaller hole first and that hole could be about the size of the two flats on the point of the large drill, or a little larger. Doing this aids in spotting the drill for accuracy to begin with and also speeds up the drilling operation for the larger drill bit considerably. After predrilling, less pressure is also required for a larger drill bit, a handy thing to know when you may have to use a hand drill rather than a drill press.
Life is good.
Oh, don't forget to punch mark your drill location, but I probably didn't have to remind you of that though. Alright, now a word about breaking through the drilled hole while drilling. This is kind of important, especially if breaking wrists or thumbs and getting bruised by spinning unclamped metal smarts too much. Whenever a drill is beginning to break through the far side of a piece of metal, always back off the pressure a bit. Many folks do the opposite and find themselves in serious trouble.