In the second picture where it have scale window, those measurements aren't that small. The hight is about 9.4 inches, and the width is about 5 inches. Your best bet to get the scaling right is to take a reference image. Look at where this piece starts on the person in the picture, and where it ends. Then measure your forearm in those same corresponding places. I would recommend measuring in centimeters and just adding a 0 on the end of the number to get millimeters. Then plug that number into the height and the other numbers will update accordingly.
Let me know if you have any further scaling issues.