Be patient and hold off with lighting and shading for a bit. We will get to that later. Let's just focus on the dimensions of your basic, run-of-the-mill pixelling hand.
The purple lines are cut in half. The yellow lines are equal lengths.The main thing I want to show is that from the base of an average thumb to the tip of an average index finger is nearly halved when your thumb is flat.
And notice that the "core" middle part of an average hand is a middle finger's length wide and a middle finger's length tall.
Finally, perhaps by coincidence, the first notch in my index finger lines up with the tip of my pinky finger.
The cool thing is that you have two hands of your own that you can use to help you build your pixel hand so that it more accurately represents an average hand!