The optimal size is a small chip, too small to be seen with the naked eye, with many year runtime. Implant it under your finger with a few touch sensing electrodes, with a 1 -> 1 million variable lumen output and a beam width from .1 to 180 degrees. Use finger controls (i.e. tap your middle finger with two other fingers to turn on, slide up and down on the side to adjust beam width, slide up and down on the front to adjust brightness).
Or, if your not into that, one approx. the size of a button. Smaller is *always* better if cost (i.e. losing it) isn't an issue.
For those of you who want it for tactile purposes, carry a lead pipe in your pocket in case you need it, but leave the flashlight on your keyring