Let's look at the male arm and it's proportions. The main thing you'll notice is that the upper arm, from the armpit to the elbow is the same length as the forearm, from the elbow to the wrist.

Since the forearm is made up of two bones - the Radius and the Ulna, that are basically laid side-by-side, the width of the wrist is wider than it is high.

Oftentimes when drawing, the elbow is not placed at the correct point on the arm. When the arm is at the side, the elbow should fall at about the navel, and the wrist should be at about the groin. The elbow should be about two and a half heads.

The elbow is not directly behind the bend in the arm in the front. It's actually a little lower than that.

Notice that when the arm is at rest, it's not perfectly straight. It can be straightened (which still does not create a perfect straight line), but it's natural at rest state has a slight bend at the elbow.