It regrows to the exact size it was, no extra skin no less skin. Unless of course it’s a serious injury but I’m talking minor skin removal. Obviously gunshot wounds, flesh eating bacteria and animal bites don’t do this because of scar tissue I assume but even with them it’s remarkable how close it comes to filling in the area. How does the body know when to stop growing the new tissue to fill it in and how come with these deeper wounds the body has a harder time figuring it out?
In: Biology
As an embryo growing from a single cell, different parts of the genome get activated in order to tell certain cells to take on different roles. As they grow to become functional parts, each segment of your finger has slightly different genetic activations which tell it “where” it is in your finger and how it is supposed to act and develop. When part gets cut off and regrows, the gradient of activated or deactivated genetic segments can be recreated and therefore “know” what type of tissue should be created and how much of it (roughly) from the patterns laid down during embryonic development for cell differentiation at the very beginning.
Obviously for larger injuries, the replacement might be too far away for this simple trick to work, but that’s how it does it on the small scale.
You might enjoy [this fun video on evolutionary development](https://youtu.be/ydqReeTV_vk) which focuses on how these changes evolve and were laid down in the first place, but also has some famous examples on how differentiation and “cells knowing what to be” works and how that can go wrong if you mess with it (like flys that grow eyes in their ass or legs on their faces if you inject different proteins into different segments during development).
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