Find two identical things – cups perhaps. Put them on a table in front of you with one about 20 cms further away from you than the other. It’s easier if they are directly on your eyeline, so maybe sit down to do this exercise. Hold up your pencil at arm’s length in front of you and close one eye. Hold the top of the pencil so that, in space, it is in line with the top of the nearest cup. Slide your thumb so that it is in line with the bottom of the cup. You now have a measurement of the cup. Move your pencil and compare this with the second cup. It is probably half the size!
This is quite a revelation to your left brain, which knows that the cups are the same size, and will refuse to ‘see’ that one now appears smaller. Unless you prove it wrong, you will tend to draw the cups the same size.
Once you have mastered this, everything to do with space and perspective become easier and easier. Using the first measurement you take (in this case the height of the cup), you can compare it to anything else in front of you. By tipping the pencil sideways you can see how wide the cup is compared to its height. This first measurement can be moved anywhere along your (flat!) picture plane to see how far things are away from each other, how big they are..... simple!
Keep your arm straight so that the distance does not change, and –as with perspective- make sure that you do not point the pencil into the picture plane. If you do that it will distort all your measurements.
You can buy anatomy books and learn by heart how many heads go into a figure, but unless you are always going to draw people standing straight as sticks, this information isn’t going to be very helpful, is it. Being able to use your pencil to measure how long an arm is compared to a head as it comes towards you in space is much more useful.