"I just want a small piece of cheese."
Translation:Je veux juste un petit morceau de fromage.
It is essentially the same error as in the English. The adverb should be next to the word it qualifies, so Je veux seulement un petit morceau. The English should be "I want only a small piece of cheese", otherwise if you say I only want etc you are saying that wanting is the only verbal action involved; you do not , eat, slice, buy, wrap or whatever the cheese, just want it.