An FU (follicular unit) must be removed individually, this requires a great deal of concentration to ensure the punch only encompasses the FU, does not overlap or cut the FU or surrounding FU´s.
Over time the preferred punch size has changed and the consensus is there is a balance between too large and too small, with around 0.75 to 1.0mm diameter punch tools being the optimum size to use. The punch size is important for a number of reasons and it all really comes back to one important aspect, the preservation of the donor area.
Larger diameter punches will remove more skin tissue; this in turn can lead to any scarring being larger and so potentially more visible. The more visible the scarring means less hair can be removed since the remaining hair has to cover the areas hair has been removed. In some cases when the donor density is high a larger punch will overlap the target FU and encompass some of an adjacent FU.
Larger punches are faster and easier to use, they require less precision and can take more hair than just a single FU. But this is not FUE in its pure form, but simply graft extraction opposed to FUE. An experienced doctor who has taken time to master FUE over the years and understand the anatomy of the skin will be using a smaller punch size; it enables the doctor to provide the patient with better results short and long term, the potential to leave a more pristine donor and over multiple procedures harvest more hair.