How would you depict the proportions of both very big and very small humanoids?
In animals, small creatures tend to have proportionally bigger eyes heads and thinner limbs, while larger creatures have proportionally smaller eyes and heads, and much thicker limbs to support heavier bodies.
How would those translate to tiny humanoids like pixies or goblins, or enormous humanoids like giants, ogres or trolls? For the latter the best I could think of was something resembling the Monsterverse version of Godzilla, with a very tiny head and huge, thick legs.
that is a pretty good rule of thumb for vertebrates!
there are limitations to it, I think. for the head size, as an example, you'd still want it to be proportionally capable of holding a brain that suits the humanoid in question, and a mouth that can eat whatever food that large body needs. and for a smaller humanoid, the head can't be so big that the body fails to support it.
you can look to hominids like homo habilis and homo floresiensis for an example of smaller humanoids, to see how the proportions change. it's not a very extreme change at that size, to be fair, but there's a difference. a more extreme example would be like, a big work horse compared to a smaller pony, or small dog breeds vs big dog breeds.
but here's a sketch of some humanoid figures at different sizes:
(image description: sketch of four humanoids in ascending sizes. To the left, there's a humanoid roughly the height of a normal human's knee, then one that comes roughly to the height of a normal human waist. the third figure is a normal human, and the final figure is about one and half times the height of a normal human. end description)