In this case this is not any prefix but just a part of the word, because the fact that there's a 'wiesz' part in here (and 'wiedzieć' in the infinitive) is just a coincidence, it seems. Okay, I found a version claiming that it can be explained by "you can say something only after you already know it". Don't know if it's true, anyway today these words aren't really connected.
"Powiedzieć" is a perfective equivalent of "mówić". See here
Almost every Polish verb has two stems (past and non-past) and two aspects (perfective and imperfective). Combining perfective aspect + non-past stem gives you the perfective future tense (e.g. powiesz). If you want to create the imperfective future tense, you'll have to use a compound (meaning combination) of the verb być and either the imperfective infinitive, or the imperfective past-stem (both work equally).
Compound (imperfective) future:
Co będziesz mówić (with infinitive).
Co będziesz mówił(a) (with past-stem).
Note that this one is irregular. The imperfective (mówić) can't be derived from the perfective (powiedzieć). You'll just have to memorize them.