Whether you are drawing a figure with an open fisted or half-fisted hand, you can determine the length of the lower arm by measuring from the elbow to the knuckles (the end of the fisted hand). Then draw the full hand from there if needed.
In either case, the following comparisons will help you get the proper proportions.
Raise your arms to your shoulders as if you’re going to do military presses. You’ll find that your knuckle line (or that of your drawn figure) lines up with the bottoms of the collarbones.
It doesn’t matter whether the hand is closed or open—promise!