I usually try to use a specific name where a more general one would be confusing (e.g., calling a qilin a qilin, not a unicorn.) Sometimes, though, specifics can obscure similarities. Kobolds, for example, have much in common with goblins--indeed, diminutive, often ugly, often chthonic creatures are found in folktales from all over the world, and where they are sufficiently similar, I am not opposed to calling them all "goblins". Some terms, though, have been so over-used that they have been rendered almost useless--fairy is one. These days, most folks seem to associate the term with tiny winged creatures like Tinkerbell, but I am accustomed to folkloric sources referring to wingless people the size of full-grown humans (or taller) as fairies (and that is the definition I favor).
Anyway, a quick rundown of the general gist of the terms as I tend to use them, in order by height:
Less than a foot tall and winged: pixie
Short and ugly: goblin
Short and not-ugly: dwarf
Short to human stature and not-beared: elf (here I disagree with Tolkien)
Human stature to tall and not-ugly: fae/fairy/sidhe, and sometimes elf. (Sidhe is the Irish term, or near to it. Fae, as far as I know, is a modern invention, but it has its uses.)
Tall and ugly: run for it.
None of this is set in stone.