It's difficult to consistently see that a PH00 is larger than a PH000. I can quite consistently eyeball the difference between a PH3 and a PH2 or a PH2 and a PH1.

As I said, it's difficult to get an accurate reading. I don't have a PH4 and I'm not sure I've seen one. The results: PH3: 0.141 inches, PH2: 0.086 inches, PH1: 0.071 inches, PH0: 0.055 inches, PH00: 0.048 inches, PH000: 0.039 inches, and PH0000 is so tiny I really couldn't get a consistent reading. I tried several different sets of screwdrivers and bits for them from different manufacturers. This is difficult to do accurately since if you insert a little more than the tip, you get a result that is too large. I tried this experiment: using some digital calipers, insert just the tip between the calipers, and measure the distance from one point on the cross to the opposite point. You could make one that widens to a meter in diameter, making it nearly impossible for humans to use. How much the tip widens to become part of the shaft is irrelevant. Based on diagrams of the tip from the original Phillips patent (shown in one of the links to ), I'm guessing that the relevant physical size is the cross you see when you point the screwdriver at your nose (as though it were a screw) and look at it end-on, or something directly related to that. The Phillips screwdriver size is based on the size at the very tip.

6 in one screwdriver drawing