Artists use various methods to measure distances accurately when working from a model or reference image, or to create realistic imagery from imagination (e.g. applying different rules of thumb regarding the relative proportions of the human figure, placement of facial features, etc.).
Often, though, fundamental drawing issues masquerade as proportion or placement errors. You draw a figure, maybe the right arm looks too long... so you shorten it, but now it looks too short. Back and forth, try as you might, you can't seem to zero in on the right length. If you are experiencing this from time to time, or frequently, chances are there are underlying drawing issues causing the element in question to appear to be the wrong size.
These two drawings use precisely the same outline. In the one on the left the boy's arm appears to be sitting tightly against his body, and his arm seems too short overall. In the one on the right, his arm appears to be extending toward the viewer, and is too long (click on the image for a larger version).
Note that even though the boy's hand in the image on the right is not larger than on the left, yet it still appears closer.
Here's a similar example. Even the ballerina's feet appear to be different lengths in the two images:
These pictures demonstrate a critical point about perception and basic drawing: shape is a more important and powerful factor in driving the perception of 3d than scale or perspective, and where an object appears to reside in 3d space has less to do with how it itself is drawn, and more to do with how it is connected to the "armature" (pun intended) that supports it. With proportion specifically, you can literally change the size of an element without actually enlarging it or reducing its 2d "footprint", by altering how it sits in space, via what's going on inside and around the given element.