Good luck if you think you can find someone who sells them to actually measure a bolt head...
As to bolt heads not measuring up to their nominal length, it's my understanding that armorers sometimes shortened them.
As to the available lengths... the 0 to 3 are what is generally reported as being the lengths used with new rifles. Armorers had access to longer bolt heads. I have a #4 as well as a #6-marked bolt head, though by my measurement the #6 is actually #5 length. The only marking on it is electropenciled, and it came to me in new condition and attached to a brand new un-serial numbered bolt... doubtless intended as an armorer's replacement part.
Of course this bolt head numbering business relates only to the No 4 & No 5 Enfields. Bolt heads also come in various lengths for the No 1 Enfield, the lengths are just a mystery to be measured. If you ever do figure out exactly what length you need, though, in my experience the folks who sell them have sorted them by the same length ranges as the number 4, and it seems to usually work out that what you get is actually 2 or 3 thousandths shorter than what you specify.
Eventually I just cut through all of that hassle by finding a guy who was selling about 20 No 1 Enfield bolts at a good price. Since then, so far at least, I've been able to find a bolt head of whatever length I was searching for, somewhere in that odd lot of bolts...