A mere reference, for those who choose to read the artist's comments! Scroll-down, dudes. Remember! Only some helpful tips! I am no authority on the matter. xD
(Also I was lazy and these drawings are horrible, I'm sorry.)
Measuring a character using heads is a very useful reference for artists. Using a head to measure is one of the earliest skills we, as artists, learn to use in formal classes.
The usual human figure is put as somewhere between seven and eight heads tall. So why do so many other variations exist? Well, each amounts of heads does serve a purpose! Each creates a unique aesthetic appeal.
One Head Tall.
This uh. This is usually only used when a character is decapitated, as it implies no body! A character usually must be bounced around, or carried (or put in a jar: see Futurama).
Two Heads Tall.
A common measurement for chibis, or extremely simplified and diminutive versions of characters used to illustrate emotions. As characters themselves, two-heads-tall characters are usually the cute characters, or animals.
Three heads tall.
A much more common measurement for animal-characters. Disney likes to use this for their own animal side-kicks, and this provides the "cute" or "childish" factor. If used for a villain, the villain is almost always comical, even if still a veritable threat. Babies are also three heads tall!
Four Heads Tall.
A common measurement for child protagonists. Many animal characters outside of Disney use these proportions. Sonic the Hedgehog, for example, is often four heads tall.
Five Heads Tall.
Again, these are usually child protagonists, though at this point many cartoon characters are about this many heads. For cartoons and likewise other abstract characters, this is a very common amount of heads to be, as it provides a larger head for more expressive emotions.
Six Heads Tall.
Many Disney heroes and heroines are six heads tall. This gives them the expressive flavor of the five-heads-tall character, while more closely resembling the technically-correct seven-or-eight-heads tall reality. Aesthetically speaking though, six heads tall is more appealing to the eye than seven heads! Ever wonder why a lot of actors are shorter than you expect? It's because shorter people tend to be closer to this six-heads ratio than taller people, which helps lend to their appeal on television.
Seven Heads Tall.
This is the lower boundary of how many heads tall a fully grown, normal human adult is.
Eight Heads Tall.
This is the upper boundary! Between this and seven-heads is generally where humans lie. Characters between these two measurements are where the more "realistic" characters will be, and many stars of graphic novels lie here. Also, a lot of really burly masculine super-heroes fall here at eight-heads-tall and above.
More Than Eight Heads?
Yes, some people do draw more than eight-heads-tall characters, and not on accident! On humans, this is a bit too tall, but this is actually the standard head-count that fashion-designers use. Ever wonder why their drawings, if you've seen them, look so tall and thin? It's not just because the models are anorexic; the drawings are nine-heads-tall. It's because nine-heads-tall is the measurement that clothes look best on, generally! It's why your clothes will never ever fit just the right way, lol.