Do the flames atop the chanukiya need to be a certain size to be valid? At the extreme, are tiny (but sustained) balls of blue flame sufficient?
A fire is a fire is a fire - See OH 673(1)
ואע"פ שאין השמנים נמשכים אחר הפתילה ואין האור נתלה יפה באותם הפתילות
Even though the wicks don't absorb the oil well and the light [fire] doesn't attach well to those wicks (--translation mine)
(i.e. even if your lights are poor, your still yotzeh the mitzvah).
That being said, the מצוה מן המבחר is to use materials that will give off a light that is "pure and clean" (Rama, ibid.). The Mishna Berura also something along the lines of what @doubleaa said: The goal here is Persumei Nisa. Hence, if people can't see them, you may have an issue at hand.