The player is 1.8 blocks tall, which I assume does not include any head-overlay pixels (the default skins do not have overlays). The player's head is 8 pixels tall (in Java Edition; Bedrock edition allows double resolution skins), and the torso and legs are each 12 pixels tall, for a total of 32 pixels.
Assuming that one block is one meter, this would put each base-layer pixel at 1.8 m / 32 = 56.25 mm. For the overlay layer, the wiki says this:
Pixel size of the 2nd layer on body, arms and legs is 0.25 pixel bigger than the skin pixel (inner layer). For head 2nd layer (Hat layer) it is 0.5 pixels bigger than the skin layer (inner layer).
That means the head overlay pixels would be 56.25 mm * 1.5 = 84.375 mm, and the rest of the body's overlay pixels would be 56.25 mm * 1.25 = 70.3125 mm. If your art is not 1:1, you can scale all these numbers up or down as needed.