When are küçük and ufak used?
Is there a difference between them?

Küçük is much more common in fact right now I cant thing of an example where ufak can't be replaced with küçük.

"küçük" can generally convey only a comperative, sometimes vague description.

you can say "an anchovy is a küçük fish", I will know it is smaller than most common fish. But that will not really give me an idea what size an anchovy can be, unless you offer further assistance.

If however, you tell me "an anchovy is an ufak fish", I would imagine anchovy as one that I can hold in my palm.

A CADDY IS "KÜÇÜK", COMPARED TO A TRAILER; BUT IT IS BY NO MEANS "UFAK".

Interestingly I have just been reading the section on diminutives in "Turkish Grammar" and, as an example, it translates these two words as:

küçük - small
ufak - tiny

That seems to make sense

Saat bire yirmi iki var (12:38)