Here is what I've got on Reddit about how long human limbs are in proportion to the lengths of their bodies:
With regard to humans, in ideal proportions, an adult is 7.5-8 heads tall. The legs (including the ankle of the foot, but not the rest) comprise 3.5-4 of those heads, or 46.7% to 50% of the total body length. The arms (not including the hands) are about 2.5 heads long, or 31.25% to a third of the total body length. A hand from its base to the tip of the middle finger is 1 head long, so if you include the hand the arms become 43.75% to 46.7% of the total body length.
Those are ideal body proportions, which is what character artists use to draw people who are visually appealing and proportionally "right." In reality, because of genetic variety, human body proportions can vary from ideal proportions quite a bit. For example, really short people have wildly different proportions.
In an alternate Earth that I've been working on, humanity is still a primate, but under a different species under a different genus under a different family. One of the ideas I've had in differentiating our humans from their humans is to change the proportions of their limbs from human to gibbon. If our limbs are identical in proportion to the length of the rest of our bodies, how long--and how short, considering gibbons--would they be?